B. In the v-t image, the displacement is equal to the "area" surrounded by the corresponding graphic line and coordinate axis. In the time of 0 ~ t 1, the displacement corresponding to the dotted line is greater than that corresponding to the solid line. From V=st, the average speed calculated by the dotted line is greater than the actual speed. Therefore, B is correct;
During the period of C.T 1 ~ T2, the dotted line reflects the uniformly accelerated linear motion, so c is wrong;
The dotted line in D, T3 ~ T4 is a straight line parallel to the time axis, and the movement reflected by this line is a straight line with uniform speed, so D is wrong;
Therefore, choose: B.