阅读短文，选择最佳答案。
The crow is an ordinary bird. It is about twenty inches (英寸) long and black all over.
Crows are dirty birds because they live on old bad food. Crows eat a lot of waste food. In this way
they are more useful to us than other birds. They clean up the dirty things in our streets. Crows are always hungry. They look for food all day long, and in the evening you can see them in large numbers flying back
to their nest in the trees. They sleep there at night.
Crows are much noisier than other birds. Very often a large number of crows will get together on one house and talk. Sometimes they talk together. And sometimes they do it by turns like human beingsc(类).
It is easy to tell from (区别) crow's sounds whether they are pleased or angry. Perhaps crows talk a lot
because they are friendly birds. A pair of crows will live together all their lives, and if one of them dies; the other one becomes very sad and quiet for the rest of its life and sometimes it dies, too.
1. A crow is_________ inches long.
A. about twenty
B. thirty
C. ten
D. less than twenty
2. Why are crows dirty?
A. They are black all over the body.
B. They like to eat dirty food.
C. They usually talk.
D. They don't have baths.
3. Crows are always___________.
A. hungry
B. pleased
C. sad
D. kind
4. Why are crows more useful to us than birds?
A. They are usually noisy.
B. They are usually dirty.
C. They are flying in large numbers.
D. They eat a lot of waste food.
5. You can tell from crows' sounds .
A. whether they are hungry or full
B. whether they are pleased or angry
C. whether they are ill or healthy
D. whether they are living or dead
，再根据抛物线的性质即可进行判断．根据图表，当x=-2，y=0，根据抛物线的对称形，当x=3时，y=0，即抛物线与x轴的交点为（-2，0）和（3，0）；∴抛物线的对称轴是直线，