什么是矿产资源？
参考答案：
矿产资源：是指天然赋存于地壳内部或地表，由地质作用形成的固态、液态、气态的具有经济价值或潜在经济价值的物质。
水电站中的压缩空气系统有几种？各有什么作用、试举例说明。
Before the child went to bed,the fater asked him to ________ all the toys he had taken out.
A．put off
B．put up
C．put away
D．put out
