My mum was cooking,_____I was reading a book.用什么词?when还是as?为什么?

问题描述:

My mum was cooking,_____I was reading a book.用什么词?when还是as?为什么?

应该填when.
根据主句与从句动作是否同时发生来区分
when可用作并列连词,表示“这时”;但as则没有类似用法.