can not 与could not的用法?

问题描述:

can not 与could not的用法?
The teacher said to us that one ____a person by his appearance.
a.can not judge b.could not judge c.isn't to judge d.doesn't judge
答案是?为什么?

语法上来讲的话COULD是CAN的过去时(但是一般过去时用CAN的特别少).
而语气上来讲,COULD要表现的委婉一些,不确保,但是可以的几率很大,CAN的意思很坚决,很肯定.