21. What information can we get from the first passage?

A. Planes go slower when they are moving with the wind.

B. It is the jet stream that affects how fast airplanes fly.

C. It takes more time to fly from NY to London than from London to NY.

D. The jet stream always blows from the east to the west across the Atlantic.

22. It can be inferred that without gravity _________.

A. buildings and other structures would float away.

B. something around your head would not fly away.

C. trees and buildings would not easily fly off.

D. everything outside buildings would fly off into space.

23. Where can we most probably read the text?

A. In a student's book. B. In a short story.

C. In a travel magazine. D. In a science fiction.

　　Have you ever been afraid? Do thunder and lightning make your heart beat faster? Maybe your mouth goes dry when your teacher says you will be given a test, or your palms（手掌） sweat when it's your turn to give your report in front of the class. Perhaps you get butterflies in your stomach when you see the bully (欺负人的人) who chooses you.

We all have fears sometimes. That's true no matter how big we are or how brave we can be. Fear of getting too close to a campfire may save you from a bad burn. And fear of getting a bad grade on a test may make you study hard. Being nervous can also sharpen your senses and help you perform better during a track meet. Some people even enjoy being a little scared. That's why they like to watch scary movies.

Usually, our bodies go into fight only when there is something to fear. However, sometimes this happens when there doesn't seem to be anything to be frightened about. When you feel scared but there doesn't seem to be a clear reason, that's called anxiety. For some kids, the feeling of anxiety or worry can happen at any time. For others, it might appear only at certain times, like when they are leaving their home