将来完成时
　　1.概念：在将来某一时刻之前开始的动作或状态
　　2.时间状语：by the time of;by the end of+时间短语（将来）;by the time+从句（将来）
　　3.基本结构：be going to/will/shall + have done
　　4例句:By the time you get back,great changes will have taken place in this area
先A
has 跟在be going to will shall 后面变动原形即have