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When running a large, low-speed, main propulsion diesel engine on heavy fuel, which of the following precautions should be observed when switching back over to diesel oil?（）
A:The diesel oil must never be allowed to mix with the heavy fuel
B:The temperature of the fuel from the pre–heater should be gradually reduced after switching over the three–way valve
C:The heating steam to the pre-heater should be secured as soon as the diesel fuel passes through the three-way valve
D:The heating steam must be secured before the diesel oil passes through the three-way valve

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出自：海船船员证

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