是it is还是what is

请看下面一道题：

_________ is hard is to do good all one’s life and never do anything bad.

A. He                        B. It                        C. That                        D. What

【分析】此题容易误选B，认为这是考查形式主语。其实，此题的正确答案是D，句首的 What is hard 是主语从句，注意 what is hard 后的谓语动词 is。假若没有此 is，则应选B，即说成 It is hard to do good all one's life and never do anything bad. 请做以下类似试题(答案均为C)：(from www.nmet168.com)

(1)  a. _________ is difficult to persuade her to stay.

b. _________ is difficult is to persuade her to stay.

A. It, It                B. What, What         C. It, What               D. What, It

(2)  a. _________ is interesting to watch children play games.

b. _________ is interesting is to watch children play games.

A. It, It                 B. What, What        C. It, What               D. What, It

(3)  a. _________ is dangerous to play with fire.

b. _________ is dangerous is to play with fire.

A. It, It                  B. What, What         C. It, What             D. What, It