单项选择题
A.a coil spring
B.a sealing ring
C.an oil distributor ring
D.main seal ring
单项选择题 The lower end of each piston rod is reduced in diameter to fit a bore in （）.
单项选择题 The lower end of each piston rod is reduced in diameter to fit a bore in （）, this being secured to the piston rod by a nut.
单项选择题 The lower end of each piston rod is reduced in diameter to fit a bore in the cross-head, this being secured to the piston rod by （）.