Read the following passage and questions, then choose the right answer from options, the answer should be one of A, B, C, D.

<passage>:
The transformation of American dentistry from drill-and-fill to shoot-and-loot is an unlikely business success story of the '90s, a case study in how a profession can work itself out of a job and still prosper. Dentists, after all, are supposed to be extinct by now. While they happily (and profitably) scraped teeth and filled cavities during the '60s and '70s, fluoride was quietly choking off their revenue stream. The percentage of children with cavities fell by half and kept falling. People stopped going to the dentist, because they didn't need to. At the same time, the government funded dental-school construction, spilling new dentists into a saturated market. Many found themselves cleaning teeth for $10 an hour in mall clinics. In 1984, Forbes magazine forecast the end of the profession. Only a few lonely dentists would survive to fill the few remaining cavities, the last vestiges of a once-great civilization on Long Island. The most important discovery dentists made was the endless vanity of aging baby boomers. "We are dealing now with the boomers who are the runners and the joggers and the dieters, and they are very concerned with how they look," says American Dental Association President Dr. Timothy Rose. Since going to the dentist was no longer a necessary evil, dentists made it an unnecessary pleasure. They allied themselves with the self-improvement movement. "You still go for the needs, for the cavity that has to get filled, but more and more people ... come here to feel better about themselves," says Dr. Stephen Friedman, a Maryland dentist. People used to be happy if they made it to old age with enough choppers to chew. But boomers, lured by media images of the Great American Smile, expect more. According to an ADA poll, the percentage of people who are "very satisfied" with their teeth has dropped from 57 percent to 46 percent in the past decade. Dentists have learned to play on this vanity and anxiety, encouraging dental care that is medically unnecessary but attractive to patients. "It's as if you went to a physician for a treatment for a disease and he said you needed a nose job," says Dr. John Dodes, author of Healthy Teeth: A User's Manual . Some dentists dress up these cosmetic measures in medical scare talk. A friend of mine just quit a dentist who was pressuring him to whiten his teeth as a "preventive measure." (To prevent what? Yellow teeth?) Many dentists claim, without scientific evidence, that the mercury in amalgam fillings is dangerous. They urge patients to replace the excellent amalgam with plastic fillings at four times the price. Dentists make a killing on bad breath--or "halitosis," as they prefer to call it. Breath clinics have sprouted up all over the country and are heavily advertised on the Web. They terrify patients with a "halimeter," a new gadget that measures a nasty smelling chemical called methyl mercaptan. Armed with the halimeter proof, the dentist then dangles expensive mouthwashes and tongue scrapers in front of the patient. Never mind that you can get the same results for free with careful brushing and basic tongue-scraping. The machine makes the sale. "Now that there is this machine that can document your complaint and can put a number on it, it motivates a patient to actually do something about it. But the treatments available now are the same ones that have been available for 15 years," says Hartel. Entrepreneurial dentists market this elective care with trained aggression. Dental management organizations often require their employees to recite a quasisales script guiding patients toward profitable cosmetics. Ads in the Journal of the American Dental Association and on the Web promote tapes and classes on marketing techniques. One person I know quit his dentist when he spied a pamphlet in the office instructing the dentist in how to get his patients to "trade up" to more expensive treatment. The ADA's annual conference is overflowing with seminars on topics such as "how to move your patients to 'yes.' " It's tempting to dismiss the whole industry as a scam, particularly when dentists keep coming up with new ailments such as bruxism (teeth grinding), periodontal disease, malocclusion (bad bite), and microcracks. But these ailments are real, and our awareness of them shows how far dentistry has come. A generation ago, dentists filled teeth and cast dentures because that's all they knew. Decay killed so many teeth that fancier problems seldom arose. Since then, researchers have studied bonding, implants, and periodontal disease. Dentists can now make crowns that last forever, bridges that stay anchored, dentures that behave almost like real teeth. A generation ago, implants were a joke. Today's implants, affixed to your jawbone by a titanium screw, can hold for the rest of your life. "If you think back a couple of generations, it was considered inevitable that people would lose their teeth when they reached midlife. Around 40 or 45, you would have your teeth taken out. Periodontal disease was not understood, and decay was rampant. But now teeth are resistant to decay and are lasting a lifetime. I have gone in 18 years from learning how to make dentures and thinking it is OK for people to lose teeth to being appalled if anyone loses teeth. It is a failure," says Dr. Judith Penski, my own fabulous D.C. dentist. Which brings us to the irony of dentistry's comeback: Just as patients love the dental care they should suspect, they resent the care they should appreciate. Aesthetic dentistry is the most profitable segment of the business because it is an easy sell. Put a camera in your mouth and you'll want whiter teeth, too. It is much harder to convince someone to poke her gums every night with a piece of rubber, to sleep with a choking plastic tooth guard, and to undergo four surgeries to fix a gum flap, all for a benefit that is decades away. The very success of dentistry has raised expectations so high that patients now object to any inconvenience. Americans under 60 believe keeping all their teeth is an entitlement: Telling them they need gum surgery to preserve their teeth makes them angry, not grateful--even though those teeth would have been goners 20 years ago. When I surveyed 100 friends and acquaintances about their dental complaints, few bitched about cosmetic dentistry that was foisted on them. They like their whiter, straighter teeth. No, they griped about the medically advisable treatments that their dentists prescribed, especially gum surgeries and mouth guards. Pity the poor dentist who abjures cosmetic dentistry but vigorously protects patients' teeth. Patients don't like periodontal treatment, so they suspect it's a rip-off. This could not be further from the truth. "Dentists are aware of providing what patients want," says Hartel. "I had a woman come in with a terrible toothache. She needed a root canal, but she did not want it. But she did want her teeth bleached, and she paid cash for it." Such is the triumph of American dentists: If they can't sell you what you need, they'll sell you what you want.

<question>:
Why does the author think that dentistry has made new discoveries in dental disease in the past decades?

<options>:
A People are more worried about their dental hygiene in recent years
B Historically, most people would lose their teeth before the more complex dental issues arose
C The doctors are making up new diseases in order to sell equipment
D To scare patients into choosing to do unnecessary cosmetic procedures

<answer>:

B