(1) An image is an S-t image, a ray passes through the origin, the distance is proportional to time, and the ratio of distance to time is a constant value, V A = S A T A = 4.8m5s = 0.964m/s;

(3) The image of B is also an S-t image, the distance is proportional to time, and the ratio of distance to time is constant, that is, VB = SbTB = 4.8m12s = 0.4m/s;

Because two cars A and B move in the same direction at the same time, car A moves faster. If car A is taken as a reference, car B moves backward.

So, the answer is: a; 0.4; Exercise.