— I think buyers ________ spend too much time going to shops.
为验证镁、铁、铜三种金属的活动性顺序,可选用的一组物质是( )
如图所示,乘坐游乐园的翻滚过山车时,质量为m的人随车在竖直平面内旋转,下列说法正确的是( )