操作人员应按照（）人员的信号进行作业，当信号不清或错误时，操作人员可拒绝执行。
A.吊装
B.安全
C.监督
D.指挥
参考答案：C
.It is the first time the students ________ to Hyde Park.
A．have gone
B．have been gone
C．had gone
D．had been gone
—______ a week do you use your computer?
—Twice.
A．How often
B．How soon
C．How many times
D．How long
