题目内容：
根据以下材料回答题：Immigration
Immigration is the act of coming to a foreign country to live. The act of leaving one's country to settle in another is called emigration. Immigrants who flee their country because of persecution, war, or such disasters as famines or epidemic are known as refugees or displaced persons (DP's). Most people find it very hard to pull up roots in their native land and move to a strange country. But throughout history, countless millions of people have done so. The heaviest immigration worldwide took place from the early 1800's to the Great Depression-the economic hard times of the 1930's. In that period, about 60 million people moved to a new land. Most came from Europe. More than half immigrated to the United States. Other destinations included Canada, Argentina, Brazil, Australia, New Zealand and South Africa. Today, the availability of fast, safe and cheap transportation helps make migration easier. Asia is replacing Europe as the major immigrant-sending area. The United States remains the chief receiving nation.
People forsake their homeland and move to another country for various reasons.
The main reason for immigration has long been economic opportunity-the lure of better land or a better job. During the 1800's, for example, the rich prairie land of the United States and Canada attracted many European farmers. Today, professional people commonly emigrated because of better opportunities elsewhere. Such emigration has sometimes been called brain drain. For example, many Philippine doctors and nurses and numerous Indian engineers and scientists have moved to the United States and Australia.
Religious persecution has led many people to move to a new land for the freedom to practice their faith. Such immigrants include Jews expelled from England in the ]200's.
Wars, revolutions, and political unrest have driven innumerable people to find new homes. In the 1990's alone, millions of refugees fled from .warfare in Iraq, Rwanda, Liberia, and Bosnia-Herzegovina.
Some immigrants were brought to a new land against their will. From the 1500's to the 1800's, Europeans shipped black Africans to the Western Hemisphere as slaves. The United Kingdom transported convicts to Australia from the late 1700's to the 1860's to relieve over crowding in British jails. Before that time, the United Kingdom sent convicts to the American colonies.
Immigrants have made enormous contributions to the culture and economy of such nations as Australia, New Zealand, Israel, Argentina, Brazil, Canada and the United States. But their accomplishments have been made with great difficulty. Many of the receiving countries have restricted immigration to maintain a homogeneous society in which all the people shared the same ethnic, geographic, and cultural background. Although some immigration laws have been relaxed, many newcomers of different backgrounds still face challenges in gaining acceptance.
Most immigrants to the United States in the nineteenth century came from__________ A．Asia
B．Europe
C．Australia
D．South Africa
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