The heart of Mexico is a high, oval valley surrounded by mountains. Once, forests covered the mountainsides, and broad, shining lakes covered nearly all the valley floor. Now, the mountains are bare and scarred with erosion , and much of the valley floor is dry and dusty. Cortez and his Spanish soldiers were the first Europeans to enter this valley. They saw the thriving Aztec city of Tenochtitlan rising from an island in one of the lakes. Surrounding it were green floating gardens. The Spaniards said it was as beautiful as a dream. Then the conquest  began. By 1521, when the Spaniards had conquered the fierce Aztecs, the island city was in ruins. Cortez decided to rebuild it after the pattern of European cities. Using the conquered Aztecs as slave laborers, Cortez built the new city, now Mexico City, in just four years. "But a generation had scarcely passed after the conquest before a sad change came over these scenes so beautiful," writes one historian. The broad, shining lakes began to dry up. Modern historians believe that Cortez began the destruction of the valley's lakes when be ordered the city rebuilt. A great deal of charcoal was needed to burn the limestone   from which cement   and mortar were made. Wood was needed to finish the interiors of the buildings. The mountainside forests were destroyed to provide the charcoal and wood. Once the sloped were bare, rainfall o longer seeped   slowly into the earth to feed the springs that filled the valley's lakes. Instead, rainfall poured off the mountainsides the city from floods, later rulers made a cut through the mountains so that the water drained away into another valley. Mexico City, once an island, had become a city of a dry plain. What is the author's purpose of writing this article?
To tell the beauty and ugliness of Mexico City To introduce the terrible change of Mexico City To explain how Mexico City changed To make people aware of the importance of nature balance