题目

 — Have you watched the video?    — No. I surfed the Internet last night only      it had been removed. A. finding           B. to be found           C. find                       D. to find 答案：D（2006•朝阳区三模）已知：在正三棱柱ABC-A1B1C1中，AB=a，AA1=2a，D、E分别是侧棱BB1和AC1的中点．（Ⅰ）求异面直线AD与A1C1所成角的余弦值；（Ⅱ）求二面角D-AC-B的大小；（Ⅲ）求证：ED⊥平面ACC1A1．