自动扶梯是（）调速的，只在启动时采用星-三角方式启动
A.需要
B.不需要
C.有时要有时不要
D.变频
参考答案：B
I shall never forget those years _______ I lived on the farm with
the farmers, _______ has a great effect on my life.
A．when; who
B．that; which
C．which; that
D．when; which
离心泵不能在低于设计流量的（）%状态下连续运行。
A、30
B、50
C、60
D、80
