[字辨] if VS whether

[pearltear ()]

請問if和whether在使用時機上有什麼不同?
有一個句子，解說的地方告訴我whether is preferred to if
→A proposal has been made to trim the horn from rhinoceroses to
discourage poachers; the question iswhether tourists
will continue to visit game parks to see rhinoceroses once the animals'
horns have been trimmed.

為什麼在這裡不選擇if呢?
查了字典，if可以接在某些動詞的後面，像是know, ask
那麼beVerb不可以嗎?



麻煩高手解答一下
謝謝!

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 218.166.58.48
推 djnospam:http://dictionary.reference.com/browse/if -> 第二個
→ djnospam:Usage Note(AHD的)