如图所示，物体A、B叠放在倾角为α=37°的斜面上．A、B的质量分别为m_A=2kg，m_B=2.5kg．A、B之间的动摩擦因数μ₁=0.5，B与斜面之间的动摩擦因数μ₂=0.4．拉着物体A的绳子沿水平方向固定在斜面顶端．现在用平行斜面向下的拉力F把B物体匀速拉动（A静止）．求

甲、乙、丙、丁四种易溶于水的物质，分别由NH₄⁺、Ba²⁺、Mg²⁺、H⁺、OH^﹣、Cl^﹣、HCO₃^﹣、SO₄^2﹣中的不同阳离子和阴离子各一种组成，将甲溶液分别与其他三种物质的溶液混合，均有白色沉淀生成，则甲为（　　）

(C)

    If someone asks you, "What colour is the sky?" I think the answer will be "blue". I'm afraid that you are wrong. Because the sky has no colour.

    What's in the sky? Is the sky full of air? I'm sure that you have asked these questions. We all know that there's air all around the world. We can not live without air. Planes can't fly very high because as they go higher, the air gets thinner. If we go far enough away from the earth, we can find that there is no air. The sky is space（太空）. In this space there is nothing but the sun, the moon and the stars.

    People always want to know more about other worlds in space. They have looked at them through （通过） telescopes （望远镜） and in this way they have known much. They know a lot about the moon now. But they want to know more about space. I'm sure they will go farther and farther until they know all about it.