Hardly ___ down ___ he stepped in.

Hardly ___ down ___ he stepped in.
A.had I sat, when B.had I sat, than
C.I had sat, when D.did I sit, than
请高手为我讲下其中的语法知识,这道题究竟该选哪个?是用一般过去时还是用过去完成时,我将感激不尽,谢谢!