By the time he retires, Carl _______ president for 15 years at t

By the time he retires, Carl _______ president for 15 years at the university.
A、would be
B、will have been
C、will be
D、has been
【正确答案】：B
【题目解析】：【考情点拨】考查by the time的用法。
【应试指导】句意：等到卡尔退休的时候，他已经在这所大学当了15年校长了。by the time引导状语从句时，从句如果用一般现在时表示将来的动作，主句用将来完成时，故选B。