一个物体受到两个力F₁和F₂的作用，两个力的大小分别是20 N和30 N，且在同一直线上，则这两个力的合力大小是（   ）

小明想了解美国的地理位置，应该查阅下列哪幅地图（　　）

根据短文阅读，选择正确答案。

    I'm Mary. This is my friend. Polly. Look! It is yellow and brown. It's fat. Its ears are small. Its eyes are big. Its tail is long. Polly likes fish and milk. It doesn't like fruit. Polly can hop and run. But it can't swim. It likes playing with a ball. I usually go to the park at the weekend(周末). Polly follows(跟随)me. I fly my kite in the park. Polly runs after me. What is Polly? Do you know? It's a cat.

健康人由于某种生理过程，使得体内形成的尿液只有原尿的1%左右，这种生理过程发生在（  ）

解决台湾问题的方针是（）