you ought to do now is to see a doctor at once.A. HowB. ThatC. WhichD. What

问题补充：

you ought to do now is to see a doctor at once.

A. How

B. That

C. Which

D. What

答案：

A. How

B. That

C. Which

(答案→)D. What

解析：考查主语从句：句意：你应该做的是立刻看医生。Is前面是主语从句，缺少宾语，用what引导主语从句，how怎么（表示方式，不能做宾语），that引导主语从句的时候，不做成分，which哪个（不符合句意），选D。