Read the following paragraph and write the topic sentence on the answer sheet. All through the long history of the Earth, waves have broken heavily against the land, and tides have pressed forward over the continents, moved back, and then returned. For no two days is the shoreline precisely the same. Not only do the tides advance and retreat in their eternal rhythms, but the level of the sea itself is never at rest. It rises or falls as the large masses of ice melt or grow, as the floor of the deep ocean basins shifts under its increasing load of sediments (沉积物), or as the Earth’s crust along the continental margins changes in adjustment to strain and tension. Today a little more land may be low to the sea, tomorrow a little less. Always the edge of the sea remains a changing boundary.
【正确答案】：Always the edge of the sea remains a changing boundary.
【题目解析】：主题句（topic sentence）是表达段落主题的句子。它用以概括段落大意，要求全段其他文字都围绕它展开。主题句通常放在段落的开端，其特点是开门见山地摆出问题，然后加以详细说明。其作用是使文章的结构更清晰，更具说服力，便于读者迅速地把握主题和想象全段的内容。主题句可以放在段中起到承上启下的作用，或放在段尾起概括全段的作用。本题中的推展句谈论的是海岸线不断变化的诸多原因，所以，最后一句是本段的主题句，起的是概括的作用。
