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單項選擇題

All three systems have the ports （） to swirl the incoming air and direct it in the appropriate path.

A.angled
B.rounded
C.being angled
D.being rounded

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單項選擇題

All the rings are slightly rounded on their external top and bottom edges to keep the oil film on the cylinder liner during （） for new piston rings.

A.the running-in period
B.the running process
C.gas exchange process
D.the boiler firing

單項選擇題

Air cocks, usually positioned at the （） in a circulating system, is used to get rid of the air in the system.

A.bending joint
B.expansion tank
C.weld
D.the highest point

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