C

    My friend John has got a restaurant(餐馆) in a big city. It is near a supermarket. He gets ten robots for the restaurant. Each robot costs (价钱为) 20,000 yuan. The restaurant is not big, but very popular（受欢迎的）. Now more and more people come here for dinner.

    The robots have different jobs in the restaurant. Two robots cook different kinds of food; one washes the dishes; one cleans the floor and the others give food to the guests (客人).

    Although robots do most of the jobs in the restaurant, John is still very busy. He must do many things to keep the robots working well. Luckily, John is good at studying robots and he knows robots very well. I think in the future, there will be a lot of robot themed restaurants (机器人主题餐厅) around the world.

（Ⅰ）证明：PA⊥BD；

（Ⅱ）若PD=AD，求二面角A﹣PB﹣C的余弦值．

植物细胞有而动物细胞没有的能量转换器是（　　）

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公共汽车遇到紧急情况急刹车，小明从座位上向前冲去。这现象是因为（　　）