Passage Two

　　Do you go to bed without brushing your teeth? You may think that not brushing is not a big deal. But caring for your mouth can prevent serious (and even embarrassing) problems.

　　Cavities happen to just about everyone. A cavity is a diseased spot in the tooth. It all starts with a sticky film made up of bacteria. That film coats your teeth and gums. The bacteria eat sugars from things you eat and drink. That produces acids. Then, the acids eat away at your teeth's hard outer coating, called enamel.

　　If you don't brush regularly to remove the film, or if you keep missing a spot, those acids keep eating away the teeth's enamel. That is how you get a cavity. "The larger it gets, the closer it gets to the nerve, increasing the chance of severe pain," says Lynn Ramer, president of the American Dentists' Association.

　　A cavity will never just go away by itself. It will only get worse. "Left untreated, 100 percent of the time a cavity will spread," says Dr. William Berlocher, a dentist who is also president of the American Academy of Pediatric Dentistry.

　　If a cavity is caught early enough, your dentist will be able to fix your tooth. The area of the tooth where the cavity was will be replaced with a special material called a filling. Some fillings are made of a metal blend containing silver. Other fillings are a tooth-colored mix of glass and a strong material called resin.

　　If you ignore a cavity, it will spread. It can go into the tooth's root. At that point, the dentist will have to do a root canal. In that procedure, the dentist removes the tooth's nerve.

　　21. According to the second paragraph, what is a cavity?

　　A. a diseased spot in the tooth

　　B. a film of acids coating the teeth

　　C. a sticky film made up of bacteria

　　D. a hard outer coating covering the teeth

　　22. What would happen if you do not brush regularly?

　　A. The film coating your teeth would be removed.

　　B. There would be less diseased spots in your teeth.

　　C. The film coating your teeth would be less sticky.

　　D. The acids would keep eating away your teeth’s hard outer coating.

　　23. If a cavity reaches the tooth’s root, how would a dentist treat it?

　　A. Replace the cavity with a filling

　　B. Replace the tooth’s enamel

　　C. Remove the tooth’s nerve

　　D. Pull out the tooth

　　24. According to the fifth paragraph, what are some fillings made of?

　　A. a mix of silver and resin

　　B. a metal blend containing glass

　　C. a metal blend containing copper

　　D. a mix of glass and resin

　　25. The main purpose of the passage is to __________.

　　A. explain the formation and treatment of cavity

　　B. emphasize the importance of brushing teeth

　　C. discuss the proper ways of brushing teeth

　　D. describe how a dentist treats a cavity