英语：Could （ ） have been ( )who helped Sunny get her work done

Anwer:
此句是一个强调句构
"It" was "they" who helped Sunny get her work done.
可以看出用"it","they"一目了然.
去掉"It was ...who"这个强调框架,原句为
They helped Sunny get her work done.
原句只不过对这个强调句构加上了推测语气,用could have done/been 表过去的推测.
It could have been they who helped Sunny get her work done.
改成问句即为原句
Could it have been they who helped Sunny get her work done?
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