原式=6+13+23+（-4.6-0.4-2.3）
=6+1-7.3
=-0.3．
Changes often happen in ______.
A．Initiating Process
B．Executing Process
C．Planning Process
D．Closing Process
下列各机械（）属于工作机
A.泵
B.透平
C.内燃机
D.通风机
