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，谢谢！
为什么要用将来完成进行时呢？将来时不就行了？

Mrs.White is your teacher.对your teacher部分的提问 52. I hear Mr and Mrs White _________ ; they ________ alone now . The white lives___,but he doesn't feel___. 请问英语高手,WHITE是什么意思? .... ( ) he was in front of Mrs Smith, turning in the watch to her. 宾语从句改错：mrs jackson says her so that he should be firendly to others. 请问:Mrs、Mr以及Mrss有什么区别？ 翻译：mrs.green had a young dog which was very vlever.he often helped her buy a newspaper. The man over there ___be Mr.White. He has gone to London. Mr.White is _old.he is _old man.(an an,/ an,an /)