The girl is often heard in the next room.A. singB. to singC. singingD. sings

问题补充：

The girl is often heard in the next room.

A. sing

B. to sing

C. singing

D. sings

答案：

A. sing

(答案→)B. to sing

C. singing

D. sings

解析：hear sb do sth表示听到某人经常做某事，变为被动语态时要把不定式符合to加上，变为be heard to do sth.