_______that time,the reform had begun to take effect,and the econ_英语解答

_______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

人气：428 ℃　时间：2019-12-16 01:15:05

解答

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.