[填空题]A serious threat to farmers in many parts of the world is erosion. Erosion occurs if a large area of land is{{U}} (36) {{/U}}of trees and is then badly treated by the farmers. The rain and wind may{{U}} (37) {{/U}}wash away, or blow away much of the topsoil. When this happens,{{U}} (38) {{/U}}of wheat or corn become weaker and weaker until nothing grows well. If erosion continues, it will turn good land into a desert. In the past when erosion{{U}} (39) {{/U}}, farmers sometimes moved to a new place to farm. Sometimes they could not move, but continued to try to grow food in the{{U}} (40) {{/U}}soil. This caused them to sink into{{U}} (41) {{/U}}. Soon some people{{U}} (42) {{/U}}that there was little rich,{{U}} (43) {{/U}}soil in the world, so the government began to try to stop erosion. One of the first experiments to stop erosion began in the United States in 1933. The experiment was carried out in the area of the Tennessee River.{{U}} (44) {{/U}}As a result