单项选择题
A.many thousand times larger than
B.the same as
C.many thousand times smaller
D.none of the above
单项选择题 The separation of impurities and water from （） is essential for good combustion of diesel engines.
单项选择题 Which of the following may hardness of the boiler water cause?（） (1) scale formation (2) tube failure (3) corrosion
单项选择题 The water level showing in a boiler water gauge glass is usually （） during working.