A
Some snakes eat other snakes, even poisonous ones. The poison does not do them any harm(伤害). Snakes can swallow(吞下) things that are bigger around than they themselves are. This is possible because the jawbones(腭骨) are loosely joined, and the mouth can stretch（伸展）to make room for a fat animal. The snake’s slim body stretches to make space for the animal when it is inside.
Many snakes eat birds’ eggs or frogs（青蛙）. Others eat pests（害虫）, mice and other animals that harm crops on farms. Because snakes do this valuable work, you should never kill one. It is too dangerous for you to kill a poisonous one. Leave that job to experts.
21. Which of the following groups of food is usually had by a snake?
A. Mice, crops, birds’ eggs.
B. Pests, frogs, some animals, plants.
C. Birds’ eggs, frogs, all animals, other snakes.
D. Mice, frogs, pests, some animals, other snakes.
22. We should not kill a snake because
A. it is poisonous B. it can do something useful for man
C. it will strike you in return D. we are not experts
23. Snakes may do some valuable work by
A. eating pests, mice and some harmful animals
B. doing harm to other animals
C. eating birds’ eggs and frogs
D. both A and C
24. In writing this passage the writer mainly wants us to
A. have a better impression(印象) about snakes.
like snakes
C. kill snakes with the help of experts
D. regard snakes as one of man’s best friends
