We found what_____then Athey did Bthey were doing

We found what (they were doing) then.
我们发现他们当时 (then) 在做什么.
A.they did B.they were doing
从句子看,只有用过去进行时态最好,如果换一个选择,可能用过去完成时态,我们发现他们做过什么.We found what they had done.但这里就不能用这个then（当时）时间状语了.