压力继电器的调定值应为（）MPa。
A、17、5～21
B、10.5～21
C、8、4～21
参考答案：A
They ____ on this project for l 4 hours, but it’s so difficult to persuade them to have a rest. [ ]
A. had worked
B. have been working
C. worked
D. are working
建设工程施工项目是由一系列相互关联、相互制约的（）所构成的。
A．作业顺序
B．作业过程
C．作业目标
D．工作流程
