问题描述：
the end of last month,he _________ in the army for one and a half years.
By the end of last month,he _________ in the army for one and a half years.
A.had joined B.had been
答案是选B,为什么不能选A?
By the end of last month,he _________ in the army for one and a half years.
A.had joined B.had been
答案是选B,为什么不能选A?
问题解答：
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