请问,Mrs.white hecame a teacher in 1985.she ___for twenty years by next summer.

请问,Mrs.white hecame a teacher in 1985.she ___for twenty years by next summer.
Mrs.white hecame a teacher in 1985.she ___ for twenty years by next summer.
a.will teach b.would have taught c.has been teaching
d.will have been teaching
我选的是d,但是答案是d 因为 我觉得是虚拟语气.到下个夏天就20年了.还没到,与现实相反.答案让我不能理解的是 她教学20年 又不是被动 为什么要加been呢?应该是will have taught才对啊,
为什么要用将来完成进行时呢？将来时不就行了？