Scientists discovered fossil evidence of mesohippus, an ancestor of the modern horse. Mesohippus lived in North America about 40 million years ago. Mesohippus was about two feet tall, ran on hooves with three toes, and had large, grinding teeth. It is believed that these early horses lived in the thick forests.

What may have helped horses change over the last 40 million years?
A) an increase in their height and weight
B) the coming and going of several ice ages
C) a change in their food, from trees to grass in the Great Plains
D) being captured by Indians and used for work, pulling wagons and riding