The boy_______ in the teachers’ office was found_______ at t

The boy_______ in the teachers' office was found_______ at the school gate y
The boy_______ in the teachers' office was found_______ at the school gateyesterday.
A.standing; smoking B.standing; to smoke
C.stood; smoking D.stood; to smoke
选哪个,为什么?

A吧.
全写应该是
the boy who is standing in the teacher's office was found smoking at the school gate yesterday.
find somebody doing something 发现某人在做什么事情.