根据提示填空（每空一词）
They were shopping at that time.(就加粗部分提问)
____ ____ they ____ at that time?

加粗部分为“做某事”，所以要用what进行提问，放句首注意首字母大写。句子时态为过去进行时，所以问句应该是“他们那时正在做什么”，表达为“What were they doing at that time?”。故本题答案为What were；doing。

略