Which of the following precautions must be taken if an electric immersion heater is used to keep the coolant in a diesel engine warm during the time the engine is secured?（）
- AThe coolant temperature must be maintained at 180℃
- BThe thermostatic bypass valves must be manually opened before the engine is started
- CThe pressure cap must be removed while the engine is secured
- DElectrical power to the heater must be secured before the cooling system is drained