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山西省2017高考英语二轮复习专题训练：阅读理解（78） 阅读下面短文，从短文后所给各题的四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项。 A The part of the earth’s history known as the coal age began more than 250 million years ago and lasted millions of years. During that time, in the area that is now the United States, coal beds formed from Texas to the north Atlantic coast. At that time thick forests swamps（沼泽） covered much of the earth’s surface. Great trees died and fell into the swamps. Then they rotted （腐烂）and new trees and plants grew on top of them. The process repeated itself for thousands of years. The rotted plants turned into a substance called peat（泥炭）, which is still forming today in many swamps. Meanwhile the land surface was changing. Movement in the earth’s crust (地壳) pushed up hills in some places, while elsewhere land sank. Swift rivers ran from the hills; heavy rains fell and the rivers overflowed, leaving muddy sand in the sunken wooded places and swamps. Oceans, too, poured in over the low-lying land, carrying sand that covered the peaty areas. The pressure of water and sand pressed the oxygen and hydrogen out of the peat, leaving carbon. Under constant pressure, the rotted material, containing a high percentage of carbon, formed gradually into coal. | 
