16.

  1. Causes of tooth decay.
  2. Methods of preventing gum disease.
  3. A new way to remove tooth decay.
  4. The breakdown of nerve tissues.

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17.

  1. A pain in the man's tooth.
  2. An article the man read.
  3. A talk with the dentist.
  4. An appointment the woman has.

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18.

  1. The man recommends the woman to his dentist.
  2. The man also once had a cavity.
  3. The woman wants to introduce the new technique to her dentist.
  4. The nerve is killed to prevent pain.

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19.

  1. It is always painful.
  2. It uses a salty-acid solution.
  3. It is conventional.
  4. It is painless.

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20.

  1. It is pain-free.
  2. It requires few tools.
  3. It deadens nerves.
  4. It takes one minute.

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M:Maria, you are leaving early today.
W:Yes, I have a dentist's appointment. I put it off as long as I could. But I know I've got at least one cavity.
M:I understand how you feel. I always felt the same until the last time I went to the dentist.
W:What changed your mind then?
M:My dentist is using a new method to clean out cavities, and it's painless.
W:Painless? You mean after you have a shot of novocaine to deaden the nerve?
M:No, I mean completely painless. The new method uses a pump that delivers a salty-acid solution to the cavity in a pulsating stream. After a few minutes, the decayed area becomes soft enough for the dentist to lift it out with a pointed tool and then the cavity is filled in the conventional way.
W:But doesn't the acid eat away good parts of the tooth or gums?
M:No. That's the best part. The acid attacks only the decay because it can't break down dentin and pulp -- the tissues that make up the healthy tooth and contain the nerves.
W:You mean no more drilling?
M:That's right. Unless cavities are too narrow to hold the filling and then the dentist must widen them by drilling.
W:I'm going to ask my dentist about this new system today. Maybe my attitude toward visits to the dentist will change too.