after the patients went into the office ,---- working

问题描述:

after the patients went into the office ,---- working
选啥啊,最好说明理由
A only a doctor did they see B only a doctor saw they
C only a doctor they saw D only a doctor had they seen

A
病人们走进办公室以后,只看到一个医生正在工作.
only放于主句句首,引起倒装,助动词要提前.