单项选择题
Cross scavenging requires the fitting of （） to prevent air or exhaust gas escape when the piston is at the top of the stroke.
A.a exhaust valve B.a piston skirt C.a piston crown D.an air suction valve
单项选择题 Crankshafts of medium speed engines are （） from a single piece of steel.
单项选择题 Cracks in the cylinders and cylinder covers may （） unequal heating.
单项选择题 Cracks in cylinder and cylinder covers may （） from unequal heating.
