HPb59-1表示平均含铜量为59％、含铅量小于1％的铅黄铜。（）
参考答案：错
Every few years, the coal workers ___________ their lungs X-rayed to ensure their health.
A．are having
B．have
C．have had
D．had had
煤和悬浮液在定压箱混合后靠自重进入旋流器，定压箱的高度（与旋流器直径）的关系约为（）
A、H≥8D
B、H≥9D
C、H≤8D
D、H≤9D
