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.