如图，ABCD为直角梯形，∠C=∠CDA=90°，AD=2BC=2CD=2，P为平面ABCD外一点，且PB⊥BD．

（1）求证：PA⊥BD；

（2）若直线l过点P，且直线l∥直线BC，试在直线l上找一点E，使得直线PC∥平面EBD；

（3）若PC⊥CD，PB=4，求四棱锥P﹣ABCD的体积．

在冬奥会滑雪比赛时，运动员用力撑雪杖使自己从山上由静止加速滑下，这表明力能使物体的____发生改变；如图所示是某运动员在滑下过程中碰撞标志杆时的情景，它说明力还能使物体的____发生改变。

① What will your dad do next week? ____     A. He is a doctor.

② Why do you like summer? ____     B. It is green.

③ What does he do? ____     C. I can see thirteen.

④ Did you have a good time? ____     D. Because I can swim.

⑤ When did you come home?____     E. We’re going to visit the art museum.

⑥ What colour is it?  ____     F. Yes, I enjoyed myself very much.

⑦ What are you going to do tomorrow? ____     G. I did my homework.

⑧ How many cows can you see?  ____     H. At about eleven in the evening.

⑨ What’s Mary doing now?  ____     I. She’s sleeping in bed. She’s ill.

⑩ What did you do last night?   ____     J. He’ll go fishing.