单项选择题
A.temperature
B.velocity
C.density
D.viscosity
单项选择题 A non-return valve in the air delivery line near to the compressor must be （）type similar to the compressor discharge valve.
单项选择题 A （） in the air delivery line near to each compressor is essential.
单项选择题 Adequate attention must be given to the water cooling system of the air compressors, for （） can cause condensation on the cylinder walls.