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
我还是不太理解C，如果不经常，park rangers不经常因为其他理由给犀牛打镇定剂，就说明是镇定剂抑制了犀牛的生育能力，这我理解，可是如果park rangers经常因为其他理由给犀牛打镇定剂，就说明不是镇定剂抑制了犀牛的生育能力吗？
-- by 会员 dengyichao (2010/3/9 15:00:36)
我终于想明白了，关键就是那个对比~如果说管理员很少使用镇定剂不是为了给犀牛重带项圈，那么老是重带项圈的犀牛就比不带的被注射的多很多，说明可能是镇定剂让母犀牛们生育能力下降：但是呢，如果管理员老用镇定剂打犀牛不是为了带项圈，那么很可能不带项圈的母犀牛用的镇定剂和戴的母犀牛一样，甚至还多，那怎么能说是镇定剂的问题呢？。。。。。。。好难想到呀~这题得好好啄米琢磨的