A、They had four toes. B、They were not as big as dogs. C、They lived in South America. D、They lived in thick forests. D

When mammals first began to grow larger, some of them evolved into the earliest ancestors of horses. (16) The first horses evolved in North America. They were about the size of dogs. They had three toes, lived in jungles and ate fruit.
    (17) Around 20 million years ago, some horses evolved to live on the grassland of central North America. These horses had one big toe and smaller side toes, and they had long legs so they could run fast to escape from tigers and wolves. They were bigger, and they had their eyes on the side of their heads, instead of in front. By about 17 million years ago, these plains horses were eating grass instead of fruit.
    Some North American plains horses used their long legs to run south to South America about 10 million years ago. About 5 million years ago, the North American plains horses evolved into modern horses. Some of them ran across the Siberian land bridge to Central Asia, where they must have been happy to find thousands of miles of grassland that they could live on. (18) Some of these Asian horses slowly spread south across Asia and Africa, evolving into donkeys. Others stayed on the Central Asian plains, where they learned to eat apples and carrots.
    Sometime around 10,000 BC, horses disappeared in both South America and North America. Probably this was because humans hunted and killed them all. In Central Asia, however, horses continued to thrive, and people tamed them about 4,000 BC.
Questions 16 to 18 are based on the passage you have just heard.
16. What can we learn about the earliest ancestors of horses?
17. What are the characteristics of horses about 20 million years ago in central North America?
18. What does the speaker say about some Asian horses?