小鸭有____只,大鸭有____只,一共有____只。
— Can you believe _________ little children can finish the difficult problem in _________ a short time?
— I can't believe it until I see it with my own eyes.
在直角坐标系中,将双曲线绕着坐标原点旋转90°后,所得到的双曲线的解析式是( )
下列关于物质性质及用途的描述中,错误的是( )