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When we can see well, we don’t think about our eyes very’ often. It is only when we cannot see perfectly that we come to see how important our eyes ale.
People who are nearsighted can only see things that are very close to their eyes. Many people who do a lot of close work, such as writing, reading and sewing, become nearsighted. Then they have to wear glasses in order to see distant things clearly.
People who are farsighted suffer from just the opposite problem. They can see things that are far away, but they have difficulty reading a book unless they hold it at arm’s length. If they want to do much reading, they must get glasses too.
Other people don’t see clearly because their eyes are not exactly the right shape. This, too, can be corrected by glasses. Some people’s eyes become cloudy because of cataracts (白内障). Long age these people often became blind. Now, however, it is possible to operate on the cataracts and r
A. Sometimes we can not see a thing because we look directly at it.
B. Sometimes we can not see a thing because we use the sides of eyes to see it.
C. The center of the eye is more sensitive than the sides of the eye after dark.
D. The center of the eye is as sensitive as the sides of the eye in daylight.

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