单项选择题
A.the cylinder cover
B.the bottom of bed-plate
C.the sump
D.the crankcase door
单项选择题 For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the （）.
单项选择题 For an ordinary modem marine four-stroke engine the inlet valves close （）.
单项选择题 For an ordinary four-stroke engine the inlet valves open （）