OG11（黄）-SC-63时态问题

63.   A proposal has been made to trim the horns from rhinoceroses to discourage poachers; the question is whether tourists will continue to visit game parks and see rhinoceroses after their horns are trimmed.

(A) whether tourists will continue to visit game parks and see rhinoceroses after their horns are

(B) whether tourists will continue to visit game parks to see one once their horns are

(C) whether tourists will continue to visit game parks to see rhinoceroses once the animals’ horns have been

(D) if tourists will continue to visit game parks and see rhinoceroses once the animals’ horns are

(E) if tourists will continue to visit game parks to see one after the animals’ horns have been

Logical predication + Diction + Verb form

The tourists are visiting for the purpose of seeing the rhinoceroses; purpose is expressed by using to, not by and. Since their could refer to either tourists or to rhinoceroses, it is inappropriately ambiguous whose horns are being trimmed. The verb following after should be the present perfect have been trimmed to reflect that the trimming must occur before the tourists arrive. When only two alternatives are possible, to continue to visit or not to continue to visit, whether (or whether or not) is properly used rather than if.

A     And see should be to see, their is ambiguous; are should be have been

B     Omitting rhinoceroses changes the meaning of the sentence; one has no referent; their absurdly and unambiguously refers to tourists; are should be have been

C     Correct. In this sentence, to correctly precedes see, it is clear that the horns belong to the animals; have been is the correct tense following once.

D     Whether is preferred to if, and see should be to see-, are should be have been

E      Whether is preferred to if, one has no referent

想问下如果不是用的have been而是are 是否也勉强可以（也就是说have been 不是一个唯一正确的答案，但是是一个更好的答案）？如果把它看作一个普通的条件从句的话，我们不是常常用一般现在时表示将来的么？

同意lu的解释

once A have been done, B.....

using "have been" to emphasize that A happened before B and A is a cause for B's happening.