Mrs. White became a teacher in 1985. She for twenty years by next summer. A. will teachB

问题补充：

Mrs. White became 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

答案：

A. will teach

B. would have taught

C. has been teaching

(答案→)D. will have been teaching

解析：语境中的时间信息：by next summer 到明年夏天，提示该题应为将来时，故排除B、C。根据句意：怀特夫人自1985年成为教师，到明年夏天就将教书法家了。因此，只有将来完成进行时才能表示这个将来某时前一直进行的动作。故选择D。