题目

_____in the queue for half an hour, the old man suddenly realized he had left the money in the car. A. Waiting                           B. To wait                    C. Having waited          D. To have waited 答案：C 解析: 略如图所示，在四棱锥S-ABCD中，BA⊥面SAD，CD⊥面SAD，SA⊥SD，且SA=SD=DC=2AB．O为AD中点．（1）求证：SO⊥BC；（2）求直线SO与面SBC所成的角．