填空题 Most people believe that the landscape is{{U}} (1) {{/U}}, but Earth is a{{U}} (2) {{/U}}body with a continually{{U}} (3) {{/U}}surface. There are 2 principal influences that shape the{{U}} (4) {{/U}}: constructive process such as{{U}} (5) {{/U}}and{{U}} (6) {{/U}}forces such as erosion. Hills and mountains are often regarded as the{{U}} (7) {{/U}}of permanence. But, interestingly enough, the higher a mountain is, the more{{U}} (8) {{/U}}it was formed. Lower mountains tend to be{{U}} (9) {{/U}}and are often the eroded{{U}} (10) {{/U}}of much higher mountain chains. The earth's crust is thought to be divided into huge, {{U}}(11) {{/U}}segments, called plates, which{{U}} (12) {{/U}}on a soft plastic layer of rocks. Some mountains are formed as a result of these crustal plates{{U}} (13) {{/U}}into each other, and forcing up the rock at the plate{{U}} (14) {{/U}}. Some mountains may be raised by{{U}} (15) {{/U}}or formed as a result of{{U}} (16) {{/U}}activity. Any landscape represents only a{{U}} (17) {{/U}}stage in the continuous{{U}} (18) {{/U}}between different forces. The main agent of erosion is{{U}} (19) {{/U}}. The landscape is continually eroded by rain, ice, tree roots and{{U}} (20) {{/U}}.