As it relates to the passage, C is better because it has an element of survival advantage. The author finds it unbelievable that apes have the capacity for language, an extraordinary advantage, and yet have never once used it. This would be comparable to unused telekinesis in humans, which would also be a tremendous survival advantage. The analogies about wings and legs represent an advantage that goes unused.
Answer choice D doesn't quite hold up. The people refusing to smoke or drink despite being able to are not neglecting a serious survival advantage. Also, the analogy isn't very good. In a way it repeats the same thing twice. It doesn't do much to explain why raising tobacco without smoking it is so unbelievable. There could be a number of uses for tobacco and alcohol.
