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 （）.

单项选择题 The lower end of each piston rod is reduced in diameter to fit a bore in the （）, this being secured to the piston rod by a nut.

单项选择题 The loss of （） from an air compressor could lead to an overheated air discharge and possibly an explosion in the pipelines leading to the air reservoir.