The reason humans switch from walking to running when they locomote at speeds exceeding 2.5 m/s is that

A) All animals use a variety of gaits
B) This is how they were taught to control gait
C) Walking at rates exceeding 2.5 m/s would require that one fall more quickly than gravity permits
D) Walking at rates exceeding 2.5 m/s would require that one fall more slowly than gravity permits