If he ___ to the teacher attentively, he ___ the answer to the problem now.

A.  had listened, would have known               B. listened, would know

C. listened, would have known                      D. had listened, would know

If he ___ to the teacher attentively, he ___ the answer to the problem now.

A. had listened, would have known 　　　　 B. listened, would know

C. listened, would have known 　　　　　　 D. had listened, would know

If he ___ to the teacher attentively, he ___ the answer to the problem now.

A. had listened, would have known B. listened, would know

C. listened, would have known D. had listened, would know