I still doubt he can overcome(克服) all the difficulties..A. whyB. whetherC. whenD. how

问题补充：

I still doubt he can overcome(克服) all the difficulties..

A. why

B. whether

C. when

D. how

答案：

A. why

(答案→)B. whether

C. when

D. how

解析：考查doubt的用法。doubt用于肯定句时用whether引导同位语从句；用于否定句时，用that引导。本句为肯定形式，故选B。