单项选择题
Corrosion inhibitors and/or soluble oils are added to diesel engine cooling systems to （）
A.maintain low pH in the cooling water B.reduce the cooling water temperature C.increase cooling water hardness D.form a protective film on metal surfaces
单项选择题 Fuel droplets injected into a diesel engine cylinder must have adequate penetration to （）
单项选择题 The primary function of a fuel delivery check valve assembly is to（）
单项选择题 For any piston ring to operate smoothly without scuffing ,the ring must be （）.
