单项选择题
Air compressor cylinder un-loaders enable the compressor to （）.
A.vary their speed according to temperature and load B.start and come up to speed before air compression begins C.change speed according to overload demands D.reduce compressed air charge density
单项选择题 The movement of heat within a fluid, caused by the application of thermal energy, is called （）.
单项选择题 A gate valve installed in a pipeline should be （）.
单项选择题 Which of the following modes of heat transfer does NOT require any physical contact between a warmer and a cooler substance?（）
