Question

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

应如何选？为什么？

Answer 1

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。

追问

还是不是很明白。不过非常感谢！

追答

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

Answer 2

四句都完全正确：

1. My mother was cooking when the telephone rang.
2. My mother was cooking at that time.
3. What were you doing when the telephone rang?
4. What were you doing at that time?

Answer 3

my mother was cooking when the telephone rang.
what were you doing at that time?

Answer 4

My mother was cooking when the telephone rang.
What were you doing when the telephone rang?
What were you doing at that time?