I don't know [ ] this evening.答案为什么选a?

问题描述:

I don't know [ ] this evening.答案为什么选a?
a.if he will be free
b.whether he is free
c.when he is free
d.when is he free

当if/whether引导条件状语从句时,用一般现在时表将来,当when引导时间状语从句时,用一般现在时表将来;但用if/whether/when 引导宾语从句时,直接用一般将来时.本句主语I,谓语don't know ,空格处应填宾语,即本句使用if...