Stroboscopic photography is a commonly used experimental method to study the motion of objects. The negative records the different positions of the object at the same time interval. The picture is as

Stroboscopic photography is a commonly used experimental method to study the motion of objects. The negative records the different positions of the object at the same time interval. The picture is as follows. As shown in the picture, the first ball moves the same distance at the same time, and the first ball moves in a straight line at a constant speed. It is in a state of equilibrium, and it is acted by a balancing force.

B ball moves different distances in the same time, and B ball moves in a variable speed straight line.

So, the answer is: a; A.