As teachers we should concern ourselves with what is said, not what we think___

答案应该是C
As teachers we should concern ourselves with what is said,not what （we think）___
we think 作插入语,去掉不影响整句话的成分.
根据句意判断,what 和 say 之间应该是被动的关系,即……被说
整句话的大意为 作为老师,我们要关心的不是学生应该说什么,而是他们说了什么.
由于说的东西和说是被动关系,首先排除D和B
其次,A选项的have在句中是错误的,正确应该为 what has to be said,而不是have
排除其它选项,答案只能是C