单选题Purging air from a hydraulic system is necessary when（）Aadding small amounts of oil to the systemBthe system has been overheatedCthe system has been drained and then filled with new oilDthe system has been idle for a long period of time
单选题
Purging air from a hydraulic system is necessary when（）
A
adding small amounts of oil to the system
B
the system has been overheated
C
the system has been drained and then filled with new oil
D
the system has been idle for a long period of time
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