1.By the time you get back,great changes___in this area.

1.By the time you get back,great changes___in this area.
为什么用问will have taken place?2.On his next birthday he __marries for ten years.用will have been 为什么不用has been?3.john _____now for the light in his room is still on .为什么选must be studying而不选must be studying?4.、I_____tennis quite well,but I have not had time to play since the new year.为什么用play不用played5.——Has sam finished his homework today?
——I have on idea .He_____it this morning.为什么选was doing不能选has done