_______that time,the reform had begun to take effect,and the economy had begun to take off .中为什么要选择by 而不选择at
问题描述：
_______that time,the reform had begun to take effect,and the economy had begun to take off .
中为什么要选择by 而不选择at
答
By that time,the reform had begun to take effect,and the economy had begun to take off .
因为后面的时态用的是过去完成时
【 by+过去时间，+过去完成时】表示到过去某一时间点，已经完成的动作
有不会的可以再问我
答
by.有“截止”之意,表示一段时间；而at.是个时间点,与后面的完成时态不相符合.