The girl was often heard ( ) happily in her room.
A.sing B.to sing C.singing D.sings
书上例句：He was heard to laugh in the next room.
有人解释说（参考中而已）不用不定式、这里为被动语态、不是过去进行时、而是现在分词doing作宾语补足语.
不是因为是被动语态,感官动词必须加上to么?
突然作个宾语补足语啥意思啊?
感官动词必须加上to（书上的原话）