判断数控机床的可靠性可参考下述（）指标。
A.机床精度
B.机床机械效率
C.机床平均无故障时间
D.机床生产率
参考答案：C
They are _____ in Shenzhen now, I think they must enjoy themselves.[ ]
A. on holiday
B. on duty
C. on business
D. on watch
简述汉长安城的布局特点。
