阅读下列短文，从每题所给的四个选项(A、B、C、D)中，选出最佳选项。
A
At first, the earth’s surface layer (层 ) was made up of huge blocks of hard, solid rocks. There slowly, the rocks broke up into smaller and smaller pieces. Finally stones, pebble(卵石) and grains of sand were form ed. Many sorts of plants and animals lived and died on the surface of the earth. They remain mixed up with the stones, pebbles and formed soil. The outmost layer of the earth is soil.
There are many ways in which large pieces of rock break into smaller ones. Wind, water, heat and cold help to break up rocks. When winds blow grains of san d against a large rock for a long time, the softer layer of the rock are slowly worn away. These leave holes and crack(裂缝) in the rock, which become bigger and finally the rock breaks up into smaller pieces.
The moving water of streams and rivers also help to break up rocks. As the water moves along, it carries with it small pieces of rock. These rub the larger ones. Thus, the larger rocks are worn down to smaller pieces.
