_______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
_______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
D 解析： 考查名词性从句。句意为：我应该说，众所周知要长学问，就得多问。句中的I should say是插入语，因此空处应填引导主语从句的连词What。