资料题：One of the reasons why black Americans could not easily mix in American society is（　）。

Many poor people made good money by selling their own living, healthy teeth.Young people's teeth were in special demand.Dentists with rich, gap-toothed patients often had severallive- tooth sellers at hand ready to have their teeth pulled.The idea was to get the best match and fit for the patient.Miss Smith's second-from-the-middle tooth might be no good.But maybe Master Brown's third-left would fit Her Ladyship's socket.Some of the double extractions took place right on the spot.The bad tooth would be hauled out.Then the newly drawn live one would be popped into the socket.Strange though it may seem these transplants sometimes worked.At least they would last for a year or two.But they, like ivory teeth, had to be tied firmly to the real teeth next to them.In some cases, neither carved teeth nor transplants worked - no matter how rich or famousthe patient.Next time you see a picture of George Washington, take a good look at it.There's something strange about the president's mouth.It has a puffed look, especially under the lowerlip.Washington suffered from poor teeth.He lost most of them when he was quite young.Dentist after dentist tried to ft him with suitable dentures.But none of them were really satisfactory.One set had dog' s teeth in the top: in the bottom were human teeth.The set was held together by strong, coiled, steel springs.ht weighed nearly a quarter of a pound.That's quite a lot of bone, lead, wax, and iron to carry round in your mouth.Yet Washington never took out his false teeth in public, even to eat.He was too proud.The artist Gilbert Stuart was asked to paint Washington's portrait.But he was troubled at the way the president' s lower lip jutted out.He told Washington to stuff wads of cotton betweenhis chin and teeth.The result was the puffed - out look we see today in Washington’s portrait onsome American bills, coins, and stamps.One set of Washington's ill-fitting false teeth is still shown in the Dental Museum in Baltimore.It is a far cry from the light, strong, well-fitted, plastic and mental dentures of today.