What he wants is his mother as quickly as he can; and what he must do is to the hospital

问题补充：

What he wants is his mother as quickly as he can; and what he must do is to the hospital at once．

A. to see; going

B. to see; go

C. see; to go

D. see; go

答案：

A. to see; going

(答案→)B. to see; go

C. see; to go

D. see; go

解析：句意：他想要的是他能尽快见到他的母亲；他必须做的是马上去医院。动词不定式（短语）作表语常用于系词be的后面。第一空是动词不定式做表语；第二空也是动词不定式做表语，但是前面已有动词do,所以后面的不定式to可以省去。如果前面没有do,那么就不能省略to,故选B。