I had picked some roses. That was when I noticed my mother's favorite vase was missing.
A.When I knew that my mother's vase was missing, I picked some roses.
B.It was not until I had picked some roses did I notice my mother's favorite vase was missing.
C.Only when I had picked some roses that I noticed my mother's favorite vase was missing.
D.I picked some roses and only then did I notice that my mother's favorite vase was missing.