单项选择题
The two-stroke cycle begins with the piston coming up from the BDC, with the scavenging ports in the sides of the cylinders being （）.
A.open B.uncover C.covered D.closed
单项选择题 Hearing a strong noise, （） at once.
单项选择题 （） that everything in the engine crankcase is correct, start up the crankcase lubricating oil pump.
单项选择题 Cooling （） the engine metals retain their mechanical properties.
