控制模式的结束方式之一是列车速度低于（）.
A.156km/h
B.10km/h
C.5km/h
参考答案：B
—By the way, do you have E-mail address?
—Sure, I think it’s useful tool to communicate with my friends.[ ]
A. an; a
B. an; an
C. a; an
D. a; a
β系数作为衡量系统风险的指标，其与收益水平负相关。( )
