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