The girl was often heard ( ) happily in her room.

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）

这个得看翻译了,有时翻译对了比语法都管用.
这个女孩常常在她房里高兴地唱着歌
你说她被别人听见了高兴的唱歌是正在唱还是已经唱完了呢?当然是正在唱,至于语法嘛,hear sb.后面只跟两种do,doing,改成被动态后是sb.be heard to do,sb be heard doing.补足语说的是补充对象的某种动作或状态,girl在做什么呢?句子给她做的补充是唱歌