Why Do Horses Tend to Move at Only Limited Ranges

Why do horses tend to move at only limited ranges of speeds?

A) They are only capable of one speed for each of their gaits-walk, trot, gallop.
B) Walking is the most energetically costly form of locomotion for horses.
C) Because of their size, horses have limited gaits and, therefore, limited speeds.
D) They locomote using the most energy-efficient combinations of gait and speed.