LSAT 1 | Section 3 | Logical Reasoning: Q21

LR Question 21 Explanation

QUESTION TEXT: The energy an animal must expend to move uphill is proportional …

QUESTION TYPE: Necessary Assumption

CONCLUSION: Small animals maintain speed uphill, while large animals don’t.

REASONING: The energy needed to go uphill increases with body weight, while the energy available increases with surface area. So small animals have an advantage.

ANALYSIS: The stimulus says:

• The energy required to go uphill increases with body weight.
• The energy available for movement increases with surface area.

From this, it concludes that small animals can move fast uphill while large animals can’t. The causal reasoning can be represented like this:

• If more body weight -> more energy needed -> moves slower
• If more surface area -> more energy available -> maintain speed

The argument applies this to small vs. large animals. But it assumes a missing link: that small animals have a higher surface area relative to body weight, meaning they have more available energy relative to what they need. Without that, the explanation doesn’t work.

___________

1. This contradicts the stimulus. The argument is based on the idea that energy needed increases with body weight, so large animals do need more energy to move uphill. So negating A just reaffirms a stated premise.
   Negation: The amount of energy needed to move uphill is greater for large animals than for small animals.
2. The conclusion isn’t about who’s faster overall, it’s about how well each maintains their speed uphill compared to flat ground. The argument compares each group to itself, not to each other.
   Negation: Small animals cannot move more rapidly than large animals.
3. CORRECT. This is the missing link. The argument assumes that small animals have more energy available relative to the energy they need. That only makes sense if small animals have a higher surface area-to-weight ratio. If the negation of C is true, the argument falls apart.
   Negation: Large animals have the same or a higher surface area-to-weight ratio than small animals.
4. The argument explains a difference between large and small animals. It doesn’t need the entire animal kingdom to behave uniformly. The argument can still hold even if there are exceptions, as long as the general trend favors small animals.
   Negation: There is significant variation in energy output to body weight across animals.
5. This contradicts the stimulus, which says energy available comes from surface area, not energy required. So this mixes the terms.
   Negation: The energy needed to run at a given speed is not proportional to surface area.