听力原文：Good morning, students. I hope you have been able to read the two books about spe

听力原文： Good morning, students. I hope you have been able to read the two books about speech and hearing problems that I put on the list. Today's lecture deals with the presence of the unusually large deaf population that existed on the Massachusetts island Martha's Vine-yard for about three centuries. From the settlement of the island in the 1640s to the 20th century, the people there, who were descended from only 25 or 30 original families, married mainly other residents of the island. They formed a highly inbred group, producing an excellent example of the genetic patterns for the inheritance of deafness. Indeed in the late 1800 one out of every 25 people in one village on the island was born deaf and the island as a whole had a deafness rate at least 17 times greater than that of the rest of the United States. Even Alexander Graham Bell, the inventor of the tele- phone and a prominent researcher into hearing loss, visited Martha's Vineyard to study the population. But because of the principles of genetics a deaf parent did not always have deaf children. In the 20th century, the local population has mixed the people off the island and the rate of deafness has fallen.

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A．A large number of its residents were deaf.

B．It was settled more than 300 years ago.

C．Each family living there had many children.

D．Alexander Graham Bell visited there.