高二英语单选helen________herkeysintheo

高二英语单选helen________herkeysintheo: helen________her ys in the office,so she had to wait until her husband___________home a had left,comes b left ,had come c had left,came d had left,would come 为什么选择C?为什么完成式用在前面而后面部用呢？如果完成式放在后边，过去式放在前面行么？为什么; 从句是过去式，主句的动作发生在重句前，因此用过去完成式。