英语单项题—Why didn't you help the little boy ?—Oh ,sorry. he had

英语单项题
—Why didn't you help the little boy ?
—Oh ,sorry. he had struggled to his feet _____I ran over
A.until B.before C.after D.since
1.before与until 的区别 2.before的可以用在任何时态吗?
_____ is known ,knowledge comes from pactice.
A.Which B.As C.It D.what
为什么不用As