1. A dental cavity is a hole in a tooth.
2. A cavity is caused by bacteria in the mouth.
3. Brushing your teeth frequently and regularly can help prevent cavities.
4. A cracked tooth can be caused by biting down on something hard.
5. Dentists treat a cracked tooth by covering it with a crown.
6. A dental x-ray is a picture of your teeth and gums.
7. An x-ray shows what can't be seen on the surface of the teeth and gums.
8. Braces are used by dentists to straighten teeth over a period of years.
9. Though braces are usually for children, they can also be used for adults with crooked teeth.
10. To extract a tooth means to remove it from the mouth.
11. Dentists have to pull hard to extract a tooth.
Dentists often inject something into the gum to block the pain during the extraction process.（前文未见生句）
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