单项选择题 Because the top of the connecting rod swings about the pin and changes direction each time the piston reaches mid-stroke, the relative speed between bearing and pin at mid-stroke is zero, （）to a maximum as the piston approaches top or bottom dead center and then （） back to zero again as the piston approaches mid-stroke and the connecting rod changes direction.

单项选择题 The slippers run （） in the crosshead guides as the piston and rod are （）and prevent the top of the connecting rod from moving sideways.

单项选择题 The connecting rob is fitted between the crosshead and the crankshaftIt transmits the firing force, and together with the crankshaft convert the （） motion to a （） motion.