单项选择题
A.can be started at low temperature and are free from preheating
B.usually are preheated by steam before being started
C.are kept at a constant temperature and free from preheating
D.are kept at a low temperature to save energy
单项选择题 In order to maintain a constant outlet water temperature of 80-85℃ from the engine, a three-way valve is installed（）
单项选择题 About the jacket water, that of the auxiliary engines usually（） is circulated by pumps, and the main engine usually （）
单项选择题 The cooling system（） needs least maintenance work for in this kind of system（）