水循环的作用？
参考答案：一方面，水通过不断转化而水质得以净化；另方面，水通过不断循环水量得以更新再生。
By the time she_____体操比赛上 at the New York Good Will Games, she_________ a dedicated junior gymnast for eleven years.
A．competed , have been
B．competes, had been
C．will compete, has been
D．competed , had been
适用于测量高温的液体、气体和蒸气的是（）温度计。
A、热电阻
B、热电偶
C、辐射
D、膨胀式
