( )we were having breakfast,he came in.A.while B.when C.afte

选A
首先排除C,要是用after,前后应当一致
对于while和when,由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导
简单点说就是 while we were having breakfast,he came in.
或者
when he came in,we were having breakfast.