题目

— ______ dress is this, Beibei? — It’s Jingjing’s. A. Who                                 B. Whose                                   C. What 答案：B如图，在直四棱柱ANCD-A1B1C1D1中，已知DC=DD1=2AD=2AB=2，AD⊥DC，AB∥DC．（1）求证：D1C⊥AC1；（2）求直线D1C与平面A1BD所成的角；（3）求点C1到平面A1BD的距离．