panbangzhu

Answer C. The key point is that "Female rhinoceroses that have been frequently recollared (shot by tranquilizer) have significantly lower fertility rates than uncollared females. Probably, therefore, some substance in the tranquilizer inhibits fertility." If it is true, we have to make sure that those "uncollared females" did not get shot by tranquilizer as often as "frequently recollared females". Otherwise, if both frequently recollared and uncollared females got shot by tranquilizer equally, we wont be able to say that the lower fertility issue is because  the tranquilizer shots.