is well known to everyone I should say is that a man becomes learned by asking question

问题补充：

is well known to everyone, I should say, is that a man becomes learned by asking questions.

A. Who

B. It

C. As

D. What

答案：

A. Who

B. It

C. As

(答案→)D. What

解析：考查主语从句:这句话有个插入语，去掉插入语后，就会看出，第二个is是谓语，前面的是主语从句，主语从句缺少主语，用what连接主语从句，句意：我要说大家都知道的事情是一个人可以通过提问来学习。选D。