吊笼停层时、操作人员应()方可进笼操作。
A、戴好安全帽
B、锁好停层保护装置
C、与司机联络
D、关掉总电源
参考答案：B
Mr Wang and his students ____________ a lesson now.[ ]
A. are having
B. had
C. have
空气中氧气在什么水平下对健康有害？（）
A．＜20.95%
B．14%～15%
C．12%
D．＜8%
E．过低或过高对健康都有害
