The head of a homing pigeon is comparatively small, but its brain is one quarter larger than that of the ordinary pigeon. The homing pigeon is very intelligent and will persevere to the point of stubbornness; some have been known to fly a hundred miles off course to avoid a storm.

　　Some homing pigeon experts claim that this bird is gifted with a form of built-in radar that helps it find its own loft after hours of flight, for hidden under the head feathers are two very sensitive ears, while the sharp, prominent eyes can see great distances in daytime.

　　Why do homing pigeons fly home? They are not unique in this inherent skill; it is found in most migratory birds, in bees, ants, toads, and even turtles, which have been known to travel hundreds of miles to return to their homes. But in the animal world, the homing pigeon alone can be trusted with its freedom and trained to carry out the missions that people demand.

　　21. This passage is mainly about_______.

　　A. homing pigeons and their training

　　B. how to buy a homing pigeon

　　C. protection of homing pigeons against the threat of extinction

　　D. liberation of homing pigeons

　　22. According to the passage, what happens to homing pigeons when they are about a month old?

　　A. They are kept in a trap.

　　B. They enter their first race.

　　C. They begin a training program.

　　D. They get their wings clipped and marked.

　　23. According to the passage, the difference between a homing pigeon and an ordinary one is_______.

　　A. the span of the wings B. the shape of the eyes

　　C. the texture of the feathers D. the size of the brain

　　24. The author mentions all of the following attributes that enable a homing pigeon to

　　return home EXCEPT_______.

　　A. instinct B. air sacs

　　C. sensitive ears D. good eyes

　　25. Why does the author mention bees, ants, toads, and turtles in the last paragraph?

　　A. To describe some unusual kinds of pets.

　　B. To measure distances traveled by various animals.

　　C. To compare their home-finding abilities with those of homing pigeons.

　　D. To interest the reader in learning about other animals.

　　答案： 21. A 22. C 23. D 24. B 25. C