液面计是用来观察容器内液面的变化的指示装置，它是根据（）原理制成的。
A.杠杆平衡
B.连通器内的液体平衡
C.物质平衡
D.水动力学
参考答案：B
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
一期梅毒的皮疹表现为（）
A.软下疳
B.扁平湿疣
C.尖锐湿疣
D.树胶肿
E.结节性梅毒疹
