單項(xiàng)選擇題Among the rings of a piston the （） one is subjected to the greatest load and temperature.

A.bottom
B.middle
C.top
D.none of the above


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1.單項(xiàng)選擇題Aluminum pistons have （） coefficient of thermal expansion compared with those of cast iron.

A.higher
B.lower
C.the same
D.none of the above

2.單項(xiàng)選擇題Although the crankshafts of medium diesel engines appear to be robust, they rely on the support of （） to develop their full strength.

A.the piston rods
B.the crank throws
C.the thrust bearings
D.the main bearings

3.單項(xiàng)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

4.單項(xiàng)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

5.單項(xiàng)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