一道LAST-弱弱地问

Test 17-I-21
The energy an animal must spend to move uphill is proportional to its body weight, whereas the animal’s energy output available to perform the task is proportional to its surface area. This is the reason that small animals, like squirrel, can run up a tree trunk almost as fast as they can move on ground level, whereas large animals tend to slow down when they are moving uphill.

Which one of the following is an assumption on which the explanation above depends?

A) The amount of energy needed to move uphill is no greater for large animals than it is for small animals
B) Small animals can move more rapidly than large animals can
C) The ratio of surface area to body weight is smaller in large animals than it is in small animals
D) There is little variation in the ratio of energy output to body weight among animals
E) The amount of energy needed to run at a given speed is proportional to the surface area of the running animal

This the reason that ……是什么意思？前面的一堆proportion是reason，还是this is the reason that…后面的内容是原因？
如何选C呢？难道是因为跑得快慢与必需的能量与可支出的能量比率（体重和身体面积的比率）决定？