A. According to the image, when Party B leaves school, the distance between Party A and Party B is above 20km, so this option is wrong;

B and A are farthest from each other in 10 minutes, so this option is correct;

C. The distance between Party A and Party B is getting smaller and smaller, so this option is wrong;

D and A ride slower than B, so this option is wrong;

So choose B.