第二部分：阅读理解（共20题，每小题3分，满分60分）
阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项，并在机读卡上将该项涂黑。
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.
56. This main idea of the passage is about the ______.
A. coal beds in the U.S.   B. earth’s history   C. formation of coal   D. swamp age
57. When plants died, they _____.
A. grew up again       B. fell into the water and rotted
C. fell into coal beds    D. turned immediately to coal
58. Hills were formed as a result of _____.
A. movements of the earth’s crust       B. the remaining from swamps
C. the remaining from swift rivers       D. ocean currents
59. What is implied but not stated in the passage?
A. Peat is still forming today.
B. The formation of coal requires high pressure.
C. The land surface changed during the coal age.
D. The formation of coal was very rapid.