题目

He ________ Mr. Black _______ to them.    A. asked ; to introduce yourself     B. asked; to introduce himself C. told ; introduce himself         D. ordered; introduce himself   答案：B已知正方体ABCD-A1B1C1D1，O是底ABCD对角线的交点．求证：（1）C1O∥面AB1D1；（2）A1C⊥面AB1D1．