Americans are a highly mobile people. What factors cause them to move? The(36)______for economic betterment is generally the mos

Americans are a highly mobile people. What factors cause them to move? The(36)desire for economic betterment is generally the most important force(37)inducing migration.
    Thus the(38)volume and direction of migration are usually determined by(39)geographic difference in economic factors, as reflected by such ones as differences in employment(40)opportunities and differences in the(41)availability and the price of land for farmers. This is not to deny the role of non-economic factors, such as climate and family ties, in(42)influencing migration. Here is a case(43)apparently showing the trend of migration in the country. When New England was declining in textile industry, there were many people moving out of it. When California has the phenomenal(显著的)growth of electronic industry, there is a large scale of inmigration to the state.
    Since 1970, there has appeared a new trend of migration in America. More Americans migrated from cities to suburbs. There are several factors for this phenomenon.(45)The construction of express highways made it possible for people to live farther away from their jobs. The telephone reduced the need for them to work in close proximity(接近)to one another. Besides these economic and technological changes, important social factors were involved. Suburban areas offered more living space than cities and less pollution. Many middle-income families now live in the suburbs.(46)This tendency prevailed in all regions except the South, where the metropolitan areas had the highest growth rate of population. Despite the new trend in migration, about 70 percent of American population are still living in metropolitan areas.