CEnT-S Practice Test: Sample Questions with Solutions

Anyone who has cycled from the countryside into a large city on a summer evening has probably noticed the change: the air seems to grow warmer street by street. This is not imagination. Built-up areas are regularly warmer than the land around them, sometimes by several degrees, and scientists have a name for the effect: the urban heat island. The name describes the shape of the problem well. On a temperature map, a city often appears as a warm patch surrounded by cooler fields and woods, like an island in a cooler sea.

The causes are well understood. Roads, car parks and many roofs are dark, and dark surfaces absorb most of the sunlight that falls on them instead of reflecting it. Materials such as asphalt, concrete and brick also store the heat they absorb for hours. In the countryside, much of the sun's energy is used to evaporate water from soil and leaves, a process that cools the surroundings; in a city, where there is less soil and there are fewer plants, this natural cooling is weaker. Tall buildings standing close together slow down the wind that could carry warm air away. Finally, cities produce heat of their own: cars, buses, factories and machines of every kind release warm air into the streets.

Many people believe that the heat island matters only on the hottest afternoons, when the sun is strongest. In fact, the difference between a city and the countryside is often largest at night. After sunset, open fields lose their heat quickly to the sky, while the streets and walls of the city go on releasing the heat they stored during the day. This is why a city can still feel warm at midnight, long after the villages around it have cooled down. Warm nights are not just uncomfortable. During a period of hot weather, the human body uses the cooler hours of the night to recover from the heat of the day. When the nights stay warm, that recovery does not happen, and the strain on the body grows from one day to the next. Older people and people with heart problems are especially at risk. For this reason, the heat island is a threat to health even in a summer whose afternoons are not unusually hot.

As a result, the most useful remedies are those that stop the city from absorbing and storing so much heat during the day. Trees are among the simplest: they shade streets and walls, and the water that evaporates from their leaves cools the air around them. Roofs and pavements in light colours reflect a large part of the sunlight back towards the sky instead of turning it into stored heat. Parks, fountains and open water work in a similar way, by giving the city back some of the natural cooling it has lost.

None of these measures needs new technology, and none of them can cool a city overnight. A young tree needs years to give real shade, and a single light roof helps only the building beneath it and the street beside it. Applied together, street by street, they can make a real difference. The heat island is not a fact of nature, like the climate of a region; it is largely the result of choices about how cities are built, and choices can be changed.

Step 1: Find the Sentence

Paragraph 3: "After sunset, open fields lose their heat quickly to the sky, while the streets and walls of the city go on releasing the heat they stored during the day. This is why a city can still feel warm at midnight…"

Step 2: Ask What "This" Explains

"This is why…" points back to a whole idea, not to one noun. What explains a warm city at midnight, "long after the villages around it have cooled down", is the contrast in the sentence before: the fields cool fast, while the city keeps giving off stored heat. Both halves are needed.

Step 3: Check the Other Options

• "Open fields lose their heat quickly": only half of the idea; fields cooling alone cannot make the city warm. Incorrect.
• "The belief that the heat island matters only on the hottest afternoons": the subject of the paragraph's first sentence, which the author rejects. Incorrect.
• "The discomfort and strain that warm nights cause": this comes after "This", in the following sentences. Incorrect.
• "The heat produced by cars, buses and factories": a cause from paragraph 2, not the idea just before "This". Incorrect.

Conclusion

$\boxed{\text{Fields cool, city keeps its heat}}$Fields cool, city keeps its heat​

Step 1: Use the Complement for "At Least One"

The opposite of "at least one die shows $5$5 or $6$6" is "neither die shows $5$5 or $6$6", so each die shows $1$1, $2$2, $3$3 or $4$4. The $36$36 ordered outcomes are equally likely.

Step 2: Apply the Values

$$\begin{aligned} P(\text{no bonus}) &= \frac{4}{6} \cdot \frac{4}{6} = \frac{4}{9} \\ P(\text{bonus}) &= 1 - \frac{4}{9} = \frac{5}{9} \end{aligned}$$P(no bonus)P(bonus)​=64​⋅64​=94​=1−94​=95​​

Check by counting: $36 - 16 = 20$36−16=20 favourable outcomes, and $\frac{20}{36} = \frac{5}{9}$3620​=95​.

Step 3: Check the Other Options

• $\dfrac{2}{3}$32​: adds $\frac{1}{3} + \frac{1}{3}$31​+31​, so the $4$4 outcomes where both dice show $5$5 or $6$6 are counted twice. Incorrect.
• $\dfrac{4}{9}$94​: this is the probability of no bonus (it forgets the final $1 - \ldots$1−…). Incorrect.
• $\dfrac{11}{21}$2111​: counts unordered pairs ($21$21 in total), but these are not equally likely: $(5;\ 6)$(5; 6) can happen in two ways, $(5;\ 5)$(5; 5) in only one. Incorrect.
• $\dfrac{1}{3}$31​: this is the probability for one die only. Incorrect.

Step 4: Final Answer

$\boxed{\dfrac{5}{9}}$95​​

Step 1: Turn "Unless" Into "If … Then"

"No $X$X unless $Y$Y" means: if not $Y$Y, then not $X$X. Here: if the customer does not show the receipt, no refund is given.

Step 2: Use the Contrapositive

Every "if … then" rule says the same as its contrapositive (swap the two parts and negate both). So the rule also says: whoever got a refund showed the receipt.

$$\begin{aligned} \text{no receipt} &\Rightarrow \text{no refund} \\ \text{refund} &\Rightarrow \text{receipt} \end{aligned}$$no receiptrefund​⇒no refund⇒receipt​

Ana got a refund, so Ana showed her receipt. Ben (receipt) and Dev (no refund) are cases the rule says nothing about.

Step 3: Check the Other Options

• "Ben got a refund": the receipt is needed, but the rule never says it is enough (converse). Incorrect.
• "Dev did not show his receipt": the inverse; Dev may have shown it and been refused for another reason. Incorrect.
• "Every customer who showed a receipt got a refund": the converse of refund $\Rightarrow$⇒ receipt. Incorrect.
• "A receipt or a refund, not both": reads "unless" as an exclusive "or"; Ana had both. Incorrect.

Conclusion

$\boxed{\text{Ana showed her receipt}}$Ana showed her receipt​

Step 1: Know What Makes a Prokaryote

A bacterium is a prokaryote: no nucleus and no membrane-bound organelles. But it still does every basic job of a cell: it has DNA (in the cytoplasm), ribosomes (smaller ones) and a plasma membrane. Jobs done by organelles in a eukaryote are done by the plasma membrane in a bacterium.

Step 2: Judge Each Statement

• Statement 1: all cells make proteins on ribosomes. Bacterial ribosomes (70S) are smaller than the ribosomes in the cytoplasm and on the rough ER of a eukaryotic cell (80S). Correct.
• Statement 2: "no mitochondria" is right, but the conclusion is wrong. E. coli respires aerobically with enzymes in its plasma membrane; it uses fermentation only when there is no oxygen. Incorrect.
• Statement 3: bacterial DNA lies free in the cytoplasm (nucleoid). In the liver cell, DNA is inside the nuclear envelope (and inside mitochondria), so it is enclosed by membranes. Correct.
• Statement 4: "only the bacterium has a wall" is right, but bacterial walls are made of peptidoglycan; cellulose walls belong to plants. Incorrect.

So statements 1 and 3 are correct.

Step 3: Check the Other Options

• "3 only": rejects statement 1, as if ribosomes were the same size in both cells, or absent in bacteria. Incorrect.
• "1 only": rejects statement 3, as if bacterial DNA had a membrane around it; it lies free in the cytoplasm. Incorrect.
• "1, 2 and 3 only": accepts that no mitochondria means no aerobic respiration; bacteria respire at the plasma membrane. Incorrect.
• "1, 3 and 4 only": accepts a cellulose wall in bacteria; their wall is peptidoglycan. Incorrect.

Step 4: Final Answer

$\boxed{\text{1 and 3 only}}$1 and 3 only​

Step 1: Look at the 1+ Ion, Not the Atom

The second electron is removed from $\text{X}^{+}$X+, so first write each $1+$1+ ion: it has one electron less than the atom.

$$\begin{aligned} \text{Na}^{+} &: 1s^{2}\,2s^{2}\,2p^{6} \\ \text{Mg}^{+} &: [\text{Ne}]\,3s^{1} \\ \text{Al}^{+} &: [\text{Ne}]\,3s^{2} \\ \text{Si}^{+} &: [\text{Ne}]\,3s^{2}\,3p^{1} \end{aligned}$$Na+Mg+Al+Si+​:1s22s22p6:[Ne]3s1:[Ne]3s2:[Ne]3s23p1​

Step 2: Na Is the Highest by Far

$\text{Na}^{+}$Na+ already has the noble-gas configuration of neon. Its next electron must come from the inner shell $n = 2$n=2: much closer to the nucleus and almost not shielded. So Na has by far the highest second ionisation energy (about $4560\ \text{kJ/mol}$4560 kJ/mol).

Step 3: The Other Three Copy the First-Energy Pattern, Moved One Place

$\text{Mg}^{+}$Mg+, $\text{Al}^{+}$Al+ and $\text{Si}^{+}$Si+ have the same outer electrons as the atoms Na, Mg and Al ($3s^{1}$3s1; $3s^{2}$3s2; $3s^{2}3p^{1}$3s23p1), each with one more proton. So they follow the first-ionisation pattern of Na, Mg, Al: the lone $3s^{1}$3s1 electron is the easiest, and a lone $3p$3p electron ($\text{Si}^{+}$Si+) is easier to remove than an electron of the full $3s^{2}$3s2 ($\text{Al}^{+}$Al+). This is the dip Al < Mg of the atoms, moved one place to the right.

$$\text{Mg}^{+} < \text{Si}^{+} < \text{Al}^{+} \ll \text{Na}^{+}$$Mg+<Si+<Al+≪Na+

(About $1450;\ 1580;\ 1820;\ 4560\ \text{kJ/mol}$1450; 1580; 1820; 4560 kJ/mol.) So the order is Mg < Si < Al < Na.

Step 4: Check the Other Options

• Na < Mg < Al < Si: the plain period trend; it misses that the second electron of Na comes from the inner shell. Incorrect.
• Na < Al < Mg < Si: the order of the first ionisation energies, not the second. Incorrect.
• Mg < Al < Si < Na: sees the jump of $\text{Na}^{+}$Na+ but misses the lone $3p$3p electron of $\text{Si}^{+}$Si+. Incorrect.
• Al < Mg < Si < Na: keeps the dip of the atoms (Al < Mg); for the $1+$1+ ions the dip moves to Si. Incorrect.

Step 5: Final Answer

$\boxed{\text{Mg < Si < Al < Na}}$Mg < Si < Al < Na​

Step 1: Square the Prefix Factor

$1\ \text{mm} = 10^{-3}\ \text{m}$1 mm=10−3 m. An area has two lengths, so the factor is used twice.

$$\begin{aligned} 1\ \text{mm}^2 &= (10^{-3}\ \text{m})^2 \\ &= 10^{-6}\ \text{m}^2 \end{aligned}$$1 mm2​=(10−3 m)2=10−6 m2​

Step 2: Convert and Write in Scientific Notation

The number in front must be between $1$1 and $10$10. Moving the comma one place to the right makes the exponent one smaller.

$$\begin{aligned} 0{,}20\ \text{mm}^2 &= 0{,}20 \times 10^{-6}\ \text{m}^2 \\ &= 2{,}0 \times 10^{-7}\ \text{m}^2 \end{aligned}$$0,20 mm2​=0,20×10−6 m2=2,0×10−7 m2​

Check: a square metre is huge compared with a wire, so the number must be very small.

Step 3: Check the Other Options

• $2{,}0 \times 10^{-4}\ \text{m}^2$2,0×10−4 m2: uses the length factor $10^{-3}$10−3 instead of $10^{-6}$10−6. Incorrect.
• $2{,}0 \times 10^{-5}\ \text{m}^2$2,0×10−5 m2: uses $10^{-4}$10−4, the factor for $\text{cm}^2$cm2, not $\text{mm}^2$mm2. Incorrect.
• $2{,}0 \times 10^{-6}\ \text{m}^2$2,0×10−6 m2: moves the comma but forgets to change the exponent. Incorrect.
• $4{,}0 \times 10^{-8}\ \text{m}^2$4,0×10−8 m2: squares the number too, $(0{,}20 \times 10^{-3})^2$(0,20×10−3)2; only the unit is squared. Incorrect.

Step 4: Final Answer

$\boxed{2{,}0 \times 10^{-7}\ \text{m}^2}$2,0×10−7 m2​