题目

I have worked for the whole term and it's a great ______ for me to be chosen as the Excellent Student Award. A . surprise B . sadness C . joy D . pity 答案：C已知在直四棱柱ABCD－A1B1C1D1中，底面ABCD为直角梯形，且满足AD⊥AB，BC∥AD，AD＝16，AB＝8，BB1＝8．E，F分别是线段A1A，BC上的点． （1）若A1E＝5，BF＝10，求证：BE∥平面A1FD．    （2）若BD⊥A1F，求三棱锥A1－AB1F的体积．