單項(xiàng)選擇題On engines having oil cooled pistons the lubricating oil pre-heater must be put into operation and the rate of heating so () that the temperature of the piston cooling oil returns are 32℃ to 35℃ over a period of at least two hours.

A.slow
B.quick
C.arranged
D.high


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2.單項(xiàng)選擇題() the oxygen content in the exhaust gas, () the combustion is.

A.The lower;the worse
B.The higher;the better
C.The lower;the better
D.The highest;the worst