is well known to everyone, I should say, is that a man bees learned by asking questions.
A. Who
B. It
C. As
D. What
网友回答
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。