题干
________ I got back, I found my mother cooking for me.
A：Since
B：Before
C：When
答案(点此获取答案解析）
C
________ I got back, I found my mother cooking for me.
A：Since
B：Before
C：When
其中
的单位是米，
的单位是秒，
秒末的瞬时速度是