问题描述：
Mr.Smith ___here for about two hours.
A.has left B.left from C.went away from D.has been away from
They have been skating __.
A.for three hours ago B.since three hours ago
C.three hours ago D.since three hours
应该选哪个?为什么 请把每道题的答案做下分析
A.has left B.left from C.went away from D.has been away from
They have been skating __.
A.for three hours ago B.since three hours ago
C.three hours ago D.since three hours
应该选哪个?为什么 请把每道题的答案做下分析
问题解答：
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