OG 63 選項C的文法疑問

[westen0331]

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

OA C

作答時我選C因為其他選項都不優
但這題我的疑問是這邊使用現在完成式have been是因為once的緣故嗎?
是否使用時間副詞的副詞子句都是要用完成式來表達未來式(本題按照句意Trim horns 應是未來式)才對?
煩請高手解惑
謝謝

[Philosophia]

不太瞭解你所謂的要用 future tense

這題是因為 for 所以要用 present perfect

這題已有他人討論過了　http://www.formosamba.com/viewtopic.php?f=11&t=20156，請遵守一題一帖的版規～

[westen0331]

不好意思我Search的時候沒有搜尋到
我的疏忽 真是很抱歉
謝謝