阅读理解
One of the least appreciated but most remarkable developments of the past 60 years is the extraordinary growth of American agriculture. Farming now accounts for about one tenth of the gross domestic product yet employs less than 1 percent of all workers. It has accomplished this feat through exceptionally high growth in productivity, which has kept prices of food low and thereby contributed to rising standards of living. Furthermore, the exportable surplus has kept the trade deficit from reaching unsupportable levels. Agriculture not only has one of the highest rates of productivity growth of all industries, but this growth appears to have accelerated during the past two decades. Over the period 1948 to 2004, total farm production went up by 166 percent. The land used for farming dropped by one quarter over the 56-year period, and investment in heavy farm equipment and other capital expenditures decreased by 12 percent. Several developments drove these changes, beginning with the replacement of the remaining horses by tractors immediately after World War II and with the expanding use of fertilizers and pesticides. Later came the adoption of hybrid seeds, genetic engineering of plants and improved livestock breeding. A key element was the U.S. Department of Agriculture's(USDA)extension service. Operating through land-grant universities and other organizations, it educated farmers on biotechnology, pest management and conservation. For many years, critics have claimed that modern agriculture is not sustainable, one of the major assertions being that it encourages erosion, which will eventually wash away most of the topsoil. Lost topsoil, the argument goes, is virtually irreplaceable because it takes up to 300 years for one inch of soil to form. But a detailed study of two large areas, the Southern Piedmont and the Northern Mississippi Valley Loess Hills, showed that based on 1982 data, soil loss has dropped sharply from the very high rates of the 1930s. The study attributed the decrease in soil erosion to the USDA, which urged farmers after World War II to adopt conservation practices such as strip cropping, whereby alternating rows are planted, and leaving plant residues in the fields year-round to inhibit water runoff. Despite being a robust contributor to the U.S. economy, modem agriculture is not without a dark side. Runoff of fertilizers, antibiotics and hormones degrade the environment and can upset the local ecology. If not grown properly, genetically modified crops could spread their DNA to conventional species.
单选题1.The growth of American agriculture has been mainly stimulated by