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问题: 问题

He had learned English well __at__ the end of six month.
我一看见had就选by了,然后怎么想都想不通
下面这句是不是就可以用by了?
He had learned English well ____ the end of June.

解答:

He had learned English well _by___ the end of June.
这句可以用by.我觉得用at也可以