问题描述：
they didn’t discover until the happened to enter the back room
They didn't discover until they happened to enter the back room_____someone had stolen the priceless painting.
A that B what C which D when
选A 怎么理解
They didn't discover until they happened to enter the back room_____someone had stolen the priceless painting.
A that B what C which D when
选A 怎么理解
问题解答：
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