A、They are very intelligent. B、They are the most intelligent. C、They are more intelligent. D、They are less intelligent. D短文最后提到，

[16]Snakes may be found just about everywhere except the North Pole region and some islands in the South Pacific Ocean. Some snakes live in warm, dry deserts. Others live in green forests, fields and woodlands. Some never go far from water and are fine swimmers.
    Some live all their lives in the sea. Most snakes can swim. [17]Because snakes are cold-blooded and depend on warm air for comfort and existence, the largest number of snakes is found in the hottest parts of the earth. Even so, many snakes stay away from the direct rays of the sun when possible. If extra hot weather comes along, snakes find a coolest place possible and stay there. In places where it freezes, snakes often hide themselves far under the ground or in caves and stay until spring.
    Snakes are smart enough to stay alive. They are smart enough to find food and kill their enemies. [18]But compared with other animals, they are believed to be below average in intelligence.
16. In which of the following places have snakes never been found?
17. Why do we find the largest number of snakes in hot regions?
18. How smart are snakes compared with other animals?