By the time he retires, professor Baker for almost forty years．
A. will have taught
B. had taught
C. has taught
D. is teaching
网友回答
(答案→)A. will have taught
B. had taught
C. has taught
D. is teaching
解析：句意：到贝克讲授退休时，他将从事教育40年了。这里教授还未退休，退休发生在将来，又因为By the time故用将来时的完成时，故选A。