My mother was cooking---.What were you doing

My mother was cooking___.What were you doing ___?A.at that time B.when the telephone rang
应如何选?为什么?

第一个空选B,第二个选A
翻译：当电话响起的时候我妈妈正在煮饭.你那时正在做什么?
at that time: 是指在过去某段特定的时间,是特指, 是“在那时”的意思,它在本题中特指when the telephone rang的那段时间.
再问： 还是不是很明白。不过非常感谢！
再答： 有点类似于代词的用法，如one作为代词时，用来指代前文提到的同类但不是同一个的可数名词 I can't find my hat. I think I must buy one.（句中的one代指前面提到的hat） My mother was cooking___.What were you doing ___?这句中横线的部分所指的时间都是相同的，也就是说可以是when the telephone rang这段时间，两个空都可以用when the telephone rang，两个空用at that time 也行得通，但避免重复一般不这样用，前一个空应该用when the telephone rang，而后一个空就用at that time来指代前面提到when the telephone rang