Our music teacher is always heard in the music room.A. singB. sangC. singingD. to sing

问题补充：

Our music teacher is always heard in the music room.

A. sing

B. sang

C. singing

D. to sing

答案：

A. sing

B. sang

C. singing

(答案→)D. to sing

解析：hear sb do sth表示听到某人经常做某事，变为被动语态时结构变为be heard to do sth.