题目

That was the first time I _______ there and I was impressed by the friendly people very much A. go          B. was going      C. had gone      D. have gone 答案：C（本小题满分14分） 如图所示，在棱长为2的正方体中，、分别为、的 中点． （1）求证：； （2）求三棱锥的体积．