,19. Where ___ you ___ ? a. have, gone b. have, leftc. did, go d. had, been答案是c为什么a不可以？

还有这个也请麻烦看一下 29. By the end of the year all but two people ____ . a. have left b. will leave c. will be leaving d. will have left 答案是d，为何选d？为何b不可以？

问的是过去的动作，用过去时。到什么时为止用完成时请认真有关时态的课，时态还未搞清楚