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
时间：2024-01-15 10:56:21
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A．真
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