Stroboscopic photography is a commonly used experimental method to study the motion of objects. The following four pictures are different objects taken by Xiaoyan with a strobe camera.

A, the object movement direction is a straight line, but the distance between two adjacent points is not equal, indicating that the object is moving at variable speed, which is irrelevant;

B, the object's movement direction is a straight line, and the distance between two adjacent points is equal, indicating that the object moves at a constant speed along a straight line and is in a state of balance, which is in line with the meaning of the question;

C, the object movement direction is a straight line, but the distance between two adjacent points is not equal, indicating that the object is moving at variable speed, which is irrelevant;

D, the direction of motion of the object is not a straight line, so it is not in equilibrium, which is irrelevant.

So choose B.