Mary was heard ____ two songs at the party.

问题描述:

Mary was heard ____ two songs at the party.
A sing
B to sing
C sang
D to have sung
应该选什么啊

hear后边不加to do排除掉bd
应该选择a
hear后应加do doing done
符合这个的只有 a ,c是过去式不是过去分词