PDH管理能力问题弱，给建立（）带来困难。
A.服务端口
B.检测系统
C.告警系统
D.集中式管理网
参考答案：D
The plane ______ when we got to the airport. [ ]
A. had taken off
B. was taken off
C. will take off
D. is taking off
一个任务单可以关联多个工作票吗？
