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.

Apologies if this has been posted before, I am still not clear on why B is not the correct answer