A writer and a lecturer ( ) into the hall while another writ

A writer and a lecturer ( ) into the hall while another writer and speaker ( ) out of it.
A.was coming;was coming B.was coming ;were coming
C.were coming;were coming D.were coming;was coming
可是前后不都是两个人吗?
为什么前面是复数,而后面是单数呢?
麻烦各位分析下