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?（）
A. The coolant temperature must be maintained at 180℃
B. The thermostatic bypass valves must be manually opened before the engine is started
C. The pressure cap must be removed while the engine is secured
D. Electrical power to the heater must be secured before the cooling system is drained
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