题目内容：
根据下面材料，回答题。Slavery has played a significant role in the history of the U．S．It existed in all the English mainland colonies and most of the Founding Fathers also had slaves，as did eight of the first 12 presidents．
Dutch traders brought 20 Africans to Jamestown，Virginia，as early as 1619，however，throughout the 17th century the number of Africans in the English mainland colonies grew very slowly．At that time，colonists used two other sources of unfree labor：Native American slaves and European indentured servants．
During those years，every colony had some Native American slaves，but their number was limited．Indian men avoided performing agricultural labor，because they viewed it as women’s work，and colonists complained that they were too“haughty”．The more important was that the settlersfound it more convenient to sell Native Americans captured in war to planters in the Caribbean than to turn them into slaves，because they often resisted and it was not hard for the slaves to escape．
Later，the policy of killing Indians or driving them away from white settlements was proposed and it contradicted with their widespread employment as slaves．
The other form of labor was the white indentured servitude．Most indentured servants consisted of poor Europeans．Desiring to escape tough conditions in Europe and take advantage of fabled opportunities in America，they traded three to seven years of their labor in exchange for the transatlantic passage．At first，it was mainly English who were the white indentured servitude but later increasingly Irish，Welsh，and German joined．They were essentially temporary slaves and most of them served as agricultural workers although some，especially in the North，were taught skilled trades．During the 17th century，they performed most of heavy labor in the Southern colonies and also consisted of the bulk of immigrants to those colonies．
At the end of the 17th century，in order to meet the labor need，landowners in America turned to African slaves．During the late 17th and 18th centuries，thanks to the dominant position of England in terms of naval superiority，English traders(some of whom lived in English America)transported millions of Africans across the Atlantic．And the transatlantic slave trade produced one of the largest forced migrations in history，blacks(the great majority of whom were slaves)increasing from about 7 percent of the American population in l680 to more than 40 percent by the middle of the l8th century．
Which of the following was true of the slavery in America? A．The colonists sold African Americans to planters in the Caribbean．
B．Native American slaves performed agricultural labor．
C．During the l7th century，the white indentured servitude was the main labor in the
Southem colonies．
D．It was at the end of the 17th century that African people began to be brought to millerica．
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