单选题Cross scavenging requires the fitting of （） to prevent air or exhaust gas escape when the piston is at the top of the stroke.Aa exhaust valveBa piston skirtCa piston crownDan air suction valve
单选题
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
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