Track and field photographer runs the line

A. When the athlete 100m, the displacement of the athlete is100m; Time is10s; ; Therefore, its average speed v = XT =10010 =10m/s; Therefore, A is correct;

B, 200m running for 20s, but the displacement is not 200m200 10, and its average speed cannot be found; So, B is wrong;

C, according to the analysis of AB, C is wrong;

D, when crossing the line, it is the speed when people reach the finish line, so it is the instantaneous speed; So, d is wrong;

So choose: a.