高程测量按所使用的仪器和施测方法不同，可分为水准测量，（），气压高程测量，其中水准测量是最准确，也是最普遍使用的。

A. the purifier driving gears are lubricated by the reclaimed
B. oil as it leaves the bowl all dirt and sludge are automatically discharged with the cooling water
C. sealing water must never be supplied until after oil is fed to the unit
D. deterioration of the bowl ring gasket will cause the purifier to lose its water seal