加热器点火时应先打开氧气额定量的1/3，再开足燃气，最后开足氧气，停顿（）s后，再用

在△ABC中，∠A=90°，∠A、∠B、∠C的对边长分别为a、b、c，则下列结论错误的是[ ]

A. a²+b²=c ²

B. b²+c²=a²

C. a²-b²=c²

D. a²-c²=b²

He didn`t tell me anything ______ he left.

[      ]

A. until

B. before

C. after

D. since

质量守恒定律的定义，化学反应前后原子的三方面没有变化分别是______、______、______．

Have you _____ your things, Bob? The train is leaving at 2:30 this afternoon, so we don't have too much time. [ ]

A. added up

B. hidden away

C. set down

D. packed up

雷达对任何高度的飞机均可进行监控。