问题描述：
____Beijing,his mother has not heard from him
A. Since she went to B After she went to
C. When she went to D.Before she went to
为什么选A,其他为什么错
A. Since she went to B After she went to
C. When she went to D.Before she went to
为什么选A,其他为什么错
问题解答：
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