B. Read the following paragraph from a college history textbook, and identify the sentence that states the main idea.
1By mid 1600, the cost of meeting the Virginia region's need for labor became a major problem. 2The country resorted to slavery to meet its growing need for labor. 3The first African blacks brought to English North America arrived on a Dutch ship and were sold at Jamestown in 1619. 4Early records are vague and incomplete, so it is not possible to say whether these Africans were treated as slaves or freed after a period of years like bond servants. 5What is certain is that by about 1640, some blacks were slaves (a few, with equal certainty, were free). 6By the 1660s, local laws had firmly established slavery in Virginia and Maryland.
—Adapted from Garraty & Carnes, The American
Nation, 10th ed., p. 42.
4. Topic sentence: ___