He was often heard ( )in his apartment before,but now,no one can hear him ( ).

问题描述:

He was often heard ( )in his apartment before,but now,no one can hear him ( ).
A.to sing;sing
B.sing;sang
C.singing;singing
D.sing;singing
并说明一下原因

选A
你看he was是被动语态be heard to do something
后面则是短语hear sb to do something
考的是被动语态和hear的用法