A、Causes of tooth decay. B、Methods of preventing gum disease. C、A new way to remove tooth decay. D、The breakdown of nerve tissue

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 no-vocaine 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.
Q4: What is the main topic of the talk?
Q5: What caused the discussion?
Q6: Which of the following is true according to the conversation?