A biker going around a circular track at constant speed is accelerating. Why?

A. Direction is constantly changing

B. Gravity is pulling on it

C. The speed us changing

D. The earth is rotating

For acceleration, there needs to be three factors that come into play. Either the change in velocity is increasing, decreasing, or it is changing direction. On a circular track, the direction is always changing. Which fits one of the three factors. So your answer would be A.