Critical Reasoning

In a certain wildlife park, park rangers are able to track the movements of many rhinoceroses because those animals wear radio collars. When, as often happens, a collar slips off, it is put back on. Putting a collar on a rhinoceros involves immobilizing the animal by shooting it with a tranquilizer dart. Female rhinoceroses that have been frequently recollared have significantly lower fertility rates than uncollared females. Probably, therefore, some substance in the tranquilizer inhibits fertility.
Which of the following is an assumption on which the argument depends?

A. The dose of tranquilizer delivered by a tranquilizer dart is large enough to give the rangers putting collars on rhinoceroses a generous margin of safety.
B. The fertility rate of uncollared female rhinoceroses in the park has been increasing in the past few decades.
C. Any stress that female rhinoceroses may suffer as a result of being immobilized and handled has little or no negative effect on their fertility.
D. The male rhinoceroses in the wildlife park do net lose their collars as often as the park's female rhinoceroses do.
E. The tranquilizer used in immobilizing rhinoceroses is the same as the tranquilizer used in working with other large mammals.

I am not fully clear on why B is not the right answer

Answer B is a trick answer that can lead you in a wrong direction.

Often on the GMAT, the makers say, in a critical reasoning answer choice, something that one might latch onto as a possible correct answer.

In this case, a comparison is being made between the fertility rates of frequently recollared females and those of uncollared females. The prompt already contains information indicating that females that have been frequently recollared have lower rates than uncollared females.

Answer choice B gives related information that is actually unnecessary for the strength of the argument. While fertility rates are discussed in the argument, the additional information, that the fertility rates of uncollared rhinoceroses have been going up, does nothing to support the argument, which depends not on any changes in fertility rates but rather on fertility rates being consistently higher in one group than in the other.