The energy an animal must expend to move uphill is proportional to its body weight, whereas the animal’s energy output available to perform this task is proportional to its surface area.
Conclusion (thus)
Small animals can run up a tree almost as fast as they move on level ground, whereas large animals slow down when moving uphill.
Evidence
The energy required to move uphill is proportional to an animal’s body weight.
The animal’s energy output available for this task is proportional to its surface area.
Evaluate
This argument uses a comparative reasoning pattern where the author explains a curious fact about animals’ differing performance on inclines. Essentially, the author is addressing why small animals maintain speed uphill while larger ones do not.
The explanation depends on an assumed link between an animal’s size and its energy capacity: namely, that the ratio of energy output (based on surface area) to energy required (based on body weight) is more favorable in small animals than in large animals.
Goal
We need an answer that supplies the missing assumption linking body weight and surface area differences between large and small animals. In other words, the argument requires that the ratio of surface area to body weight is lower in larger animals compared to smaller ones, which explains why large animals are less able to sustain high speeds uphill.
Correct
This answer choice provides the necessary link by assuming that the ratio of surface area to body weight is smaller in large animals, meaning that larger animals get relatively less available energy (per unit of weight) for moving uphill. Without this assumption, the explanation for why large animals slow down uphill would fall apart.
Irrelevant Comparison
This answer choice claims that the amount of energy needed to move uphill is no greater for large animals than for small animals. However, the argument already establishes that energy required depends on body weight; it does not assume that this requirement is equal across sizes.