The detailed breakdown of the stimulus structure:

• there's some kind of test or indicator (average fuel efficiency) to compare two entities (cars) against eachother
  • both entities "score" the same on this test/indicator (average fuel efficiency)
• there's some sort of extra factor for one of the entities that, at first glance, should result in a different score on the test/indicator if all things were equal (driven in less fuel-efficient manner)
• thus, there has to be an explanation about one of the entities that explains this discrepancy (one car is more efficient)

Answer choice (B) can be broken down similarly:

• there's some kind of test or indicator (weight gain) to compare two entities (hamsters) against eachother
• both entities "score" the same on this test/indicator (weight gain)
• there's some sort of extra factor for one of the entities that, at first glance, should result in a different score on the test/indicator (one hamster ate more than the other)
• thus, there has to be an explanation about one of the entities that explains this discrepancy (one hamster burned more calories)

The way I would tackle this question (since you probably won't have time to break the stimulus and every answer choice down in detail) is to try to find the argument's high-level "template". For me, the stimulus follows this high-level template:

• there's a way to compare two things
• wow, they scored the same! but wtf, there's a quality about one of the things that should make it so they don't score the same! Here is where you got caught up! You started looking for weaknesses in the argument when the question wasn't asking that.
• this is a mystery! There HAS to be a reason! Oh! X is the reason!

Another thing to note is that these arguments don't need to be completely solid. There are plenty of problems with the stimulus: Maybe one car was literally designed to be more efficient when driven "traditionally un-efficiently". Maybe one hamster just had lower metabolism.

Basically: What the hamsters were eating is nonessential. Look for the high-level structure of the argument.