A 2.0-kg block is attached to one end of a spring with a spring constant of 100 N/m and a 4.0-kg block is attached to the other end. The blocks are placed on a horizontal friction less surface and set into motion. At one instant the 2.0-kg block is observed to be traveling to the right with a speed of 0.50 m/ and the 4.0-kg block is observed to be traveling to the left with a speed of 0.30 m/. Since the only forces on the blocks are the force of gravity, the normal force of the surface, and the force of the spring, we conclude that:
A. the spring is compressed at the time of the observation
B. the spring is not compressed at the time of observation
C. the motion was started with the masses at rest
D. the motion was started with at least one of masses moving
E. the motion was started by compressing the spring
