According to the image, after 6 seconds, the distance between car A and car B is 0.6m and =0.3m. Because the two cars are moving in a straight line at the same time and place, when the two cars are moving for 6 seconds, the distance between the two workshops is equal to the sum of the distances between the two cars, that is, car A+car B =0.6m+0.3m = 0.9m, so A and B are wrong and C is correct.

Because car B moves in a straight line at a uniform speed, the speed and mass of car B remain unchanged, and the kinetic energy of car B remains unchanged, so D is wrong.

So choose C.