题目

It's a lovely dress but it's too expensive. I can't ________it. A . spend B . pay C . afford D . cost 答案：C（2012•广安二模）如图，在三棱锥P-ABC中，PB⊥面ABC，∠ABC=90°，AB=BC=2，∠PAB=45°，点D，E，F分别是AC，AB，BC的中点．（1）求证：EF⊥PD；（2）求直线PF与平面PBD所成的角的大小；（3）求二面角E-PF-B的大小．