关于OG对whether和if用法的疑惑，请指教

看了CD上面很多关于whether和if的用法比较，但是没能解开我对OG中一段解释的疑惑，请大家指教。

OG 11-94

236. The Coast Guard is conducting tests to see whether pigeons can be trained to help find survivors of wrecks at sea.

(A)  to see whether pigeons can be trained to help find

(B)  to see whether pigeons can be trained as help to find

(C)  to see if pigeons can be trained for helping to find

(D)  that see if pigeons are able to be trained in
                helping to find

(E)   that see whether pigeons are able to be trained for help in finding

对于正确答案没有任何异议，但是看解释的时候，里面有这么一句话"whether is preferred to if when there are only two alternatives" 请问，应该怎么理解这句话呢？当他说只有there are only two alternatives的时候，是不是包含了所以“是否。。。”的从句？谢谢

看到if表达是否的含义直接毙掉，选whether，别想其他理由，考试没那么多时间。