单选题：根据下列文章，回答36～40题。In 1784, five years before he became preside

根据下列文章，回答36～40题。In 1784, five years before he became president of the United States, George Washington, 52, was nearly toothless. So he hired a dentist to transplant nine teeth into his jaw - having extracted them from the mouths of his slaves.
　　That's a far different image from the cherry-tree-chopping George most people remember from their history books. But recently, many historians have begun to focus on the roles slavery played in the lives of the founding generation. They have been spurred in part by DNA evidence made available in 1998, which almost certainly proved Thomas Jefferson had fathered at least one child with his slave Sally Hemings. And only over the past 30 years have scholars examined history from the bottom up. Works of several historians reveal the moral compromises made by the nation's early leaders and the fragile nature of the country's infancy. More significantly, they argue that many of the Founding Fathers knew slavery was wrong - and yet most did little to fight it.
　　More than anything, the historians say, the founders were hampered by the culture of their time. While Washington and Jefferson privately expressed distaste for slavery, they also understood that it was part of the political and economic bedrock of the country they helped to create.
　　For one thing, the South could not afford to part with its slaves. Owning slaves was "like having a large bank account," says Wiencek, author of An Imperfect God: George Washington, His Slaves, and the Creation of America. The southern states would not have signed the Constitution without protections for the "peculiar institution," including a clause that counted a slave as three fifths of a man for purposes of congressional representation.
　　And the statesmen's political lives depended on slavery. The three-fifths formula handed Jefferson his narrow victory in the presidential election of 1800 by inflating the votes of the southern states in the Electoral College. Once in office, Jefferson extended slavery with the Louisiana Purchase in 1803; the new land was carved into 13 states, including three slave states.
　　Still, Jefferson freed Hemings's children - though not Hemings herself or his approximately 150 other slaves. Washington, who had begun to believe that all men were created equal after observing the bravery of the black soldiers during the Revolutionary War, overcame the strong opposition of his relatives to grant his slaves their freedom in his will. Only a decade earlier, such an act would have required legislative approval in Virginia.George Washington's dental surgery is mentioned to A．show the primitive medical practice in the pas
B．demonstrate the cruelty of slavery in his day
C．stress the role of slaves in the histor
D．reveal some unknown aspect of his lif