单选题In an engine with air inlet valve and exhaust valve on its cylinder heads the fresh air will enter into the cylinder （） and the gases exhaust （）, there being an adequate overlap between the inlet opening and the exhaust closing.Adownwards；upwardsBupwards；downwardsCdownwards；downwardsDupwards；upwards
单选题
In an engine with air inlet valve and exhaust valve on its cylinder heads the fresh air will enter into the cylinder （） and the gases exhaust （）, there being an adequate overlap between the inlet opening and the exhaust closing.
A
downwards；upwards
B
upwards；downwards
C
downwards；downwards
D
upwards；upwards
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