题目

Her mother is ___________. She ____________ a car now.          A. a driver, drive B. driver, drives C. a driver, is driving 答案：C已知斜三棱柱ABC-A1B1C1的底面是直角三角形，∠ACB=90°，侧棱与底面所成角为θ，点B1在底面上的射影D落在BC上．（1）求证：AC⊥平面BB1C1C；（2）若，且当AC=BC=AA1=3时，求二面角C-AB-C1的大小．