读一读,做出相对应的回答
①Good afternoon! ____ A. Good afternoon!
②Here you are. ____ B. Hi! Ni Ni.
③Hello! ____ C. Thank you!
④I’m Ni Ni! ____ D. Hi!
(Ⅰ)证明:PA⊥BD;
(Ⅱ)若PD=AD,求二面角A﹣PB﹣C的余弦值.
两幅图分别是两条大河的河口图,图中小岛因泥沙不断堆积而扩展,最终将与河的哪岸相连( )
①甲岸 ②乙岸 ③丙岸 ④丁岸