I / arrive / 7 am/ the post office / open

A. When I arrived at 7 a.m., the post office which had just been opened.

B. When I arrived at 7 a.m., the post office had just been opened.

C. I arrived at 7 a.m. when the post office had just been opened.

D. I arrived at 7 a.m. at the time the post office had just opened.