B
其实C 也是对的.
不定式作表语时一般不可以省去to,但：
当主语部分有do,作“做”讲,表语陈述做的事时,不定式符号to 可以省去,也可以不省
如：
The first thing that you shoud do is (to) improve your English.
What you should do is (to) help them out
The first thing for you to do is (to) find out when the train leaves.
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