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?（）
- AThe diesel oil must never be allowed to mix with the heavy fuel
- BThe temperature of the fuel from the pre–heater should be gradually reduced after switching over the three–way valve
- CThe heating steam to the pre-heater should be secured as soon as the diesel fuel passes through the three-way valve
- DThe heating steam must be secured before the diesel oil passes through the three-way valve