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.
In evaluating the argument, it would be most useful to determine which of the following?
A. Whether there are more collared female rhinoceroses than uncollared female rhinoceroses in the park
B. How the tranquilizer that is used for immobilizing rhinoceroses differs, if at all, from tranquilizers used in working with other large mammals
C. How often park rangers need to use tranquilizer darts to immobilize rhinoceroses for reasons other than attaching radio collars
D. Whether male rhinoceroses in the wildlife park lose their collars any more often than the park’s female rhinoceroses do
E. Whether radio collars are the only practical means that park rangers have for tracking the movements of rhinoceroses in the park
首先分析一下题干：
背景信息：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.动物园的管理人员要在犀牛身上安装一种collar来记录犀牛的活动。如果collar滑掉了，还要再按上。
Evidence：
Putting a collar on a rhinoceros involves immobilizing the animal by shooting it with a tranquilizer dart. 安装collar要打镇定剂。
Female rhinoceroses that have been frequently recollared have significantly lower fertility rates than uncollared females。又被重新安装collar的母犀牛的生育能力比没有安装collar的生育能力低。
Conclusion：
probably，some substance in the tranquilizer inhibits fertility。镇静剂影响生育能力。
