自动扶梯以匀速自下向上行驶，甲每秒钟向上走1级梯，乙每秒钟向上走2级梯，结果甲30秒到达梯顶，乙20秒到达梯顶，该扶梯共有（）级。

A． 40

B．60

C．80

D．100

Personnel working aloft (above 2metres) should wear（）or other arresting device at all times.

A.a safety harness with lifeline

B.a safety net

C.a life buoy

D.buoyancy agarments