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.

A.accelerates；decelerates
B.accelerates；accelerates
C.decelerates；decelerates
D.decelerates；accelerates