The Fossil Record Indicates That the Earliest Known Ancestor of Horses

The fossil record indicates that the earliest known ancestor of horses has four toes.Modern horses have only one.Which of the following best explains why this occurred?

A) Environmental pressures selected for four toes over one dominant central toe over millions of years.
B) Over the course of a lifetime,ancestors would lose toes due to the stresses of prairie life,such as predation and injury.
C) Because the ancestors did not use all of their toes,their offspring were born with fewer toes.
D) Gradually,over the course of many generations,the four toes merged together to form a single toe.
E) Offspring with genetic mutations resulting in fewer toes were more likely to survive and reproduce.