阅读理解。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。
Recognized as the largest desert in the world, the Sahara runs across North Africa, Covering over 3 million square miles from the Atlantic Coast to the Nile River Valley. Except for an occasional oasis(绿洲), this vast land is mainly made up of sand, stones, and worn out mountains. The burning sun causes daytime temperatures to rise to 70 degrees in the shade, and days without rain commonly last for years.
Paintings found on the walls of caves in the Sahara Desert suggest that a few thousand years ago, large quantities of plants grew there and covered the desert floor. Many curious and unusual insects, birds and animals got enough food that allowed them to grow and reproduce. At that time, huge river systems and many oases supported growing communities(社团，社区) where a large number of people lived.
Research scientists believe that the land became a desert for several reasons. For centuries people there have wandered from place to place in search of food, water and grazing(吃草)land for their animals. These people have always considered the number of animals they have as a sign of wealth. It is considered more important to have 100 sick and starving animals than 15 healthy ones. The large herds (群)have been allowed to go freely and graze on desert plants and grass. Over time, large sections of land have been left uncovered. In addition to allowing overgrazing, these people cut down whatever trees they could find and used them for firewood. The sand was unable to hold on to the great heat of the day, so nightting temperatures often drop below freezing and nothing was left to hold the soil together.
Scientists have been studying different ways to bring back the green desert. Through research and experiments, they hope to be able to produce plants that will once again support the life there. Huge holes filled with much water lie under the desert surface. Scientists believe this rainwater simply flew down through the sand over hundreds of years and collected in the big, rocky holding tanks. By making use of the sun’s power, scientists in the Sahara have made experiments in which they got huge amount of electricity. This electricity was then used to operate drilling machines and pumps to pull the rainwater from the underground tanks. By drilling the hole one mile deep into the floor of the desert, one machine was able to recover more than 80 gallons of ice—cold water each second, or 288,000 gallons per hour.
Scientists have had some success turning this wasteland into useful farmland. Today the desert is dotted with huge wheat fields watered by a system of pipes that carry water from the underground tanks. Farmers have also learnt that plants grow better if seeds are planted next to stones. Studies show that stones provide protection from the sun, allowing seeds to grow in a shady and cool environment. In addition to that, greenhouses have been built to protect plants from the sunshine so that less water is needed. Another idea is to grow plants that can live on a diet of salt water, since the ocean is so near to the desert.
1. What will probably happen if the local people continue their customs and traditions?
A. The desert will continue to be enlarged.
B. Tourists will begin traveling to North Africa.
C. More oases will begin to appear across the desert.
D. The number of healthy animals will become larger.
2. Which of the flowing statements is a fact presented in the passage?
A. The desert is rich in beautiful color and scenery.
B. Camels can go for days without drinking any water.
C. The Sahara will one day become a beautiful garden.
D. Large river systems once supported many communities.
3. The author of the passage provides evidences (证据)that scientists are .
A. pleased by the progress but do not have the money to continue
B. discouraged by the poor results of their experiments
C. hopeful about the chances of life coming back to the desert
D. troubled by the lack of water found underground
4. Which of the following is the best summary of this passage?
A. Drilling machines help to water the desert floor and make plants and animals alive.
B.Scientists have been able to turn this large wasteland into a successful farming community.
C. So much soil has become sand that the desert will never be able to support life as it once did.
D. The desert no longer supports plenty of life, but researchers hope to change it with new scientific techniques.
1、答案解析：答案为A。本题为推理题。结合前三段可知，撒哈拉沙漠原本是绿洲，后由于人类过度放牧(overgrazing)、滥砍乱伐(these people cut down whatever trees they could find )导致了沙漠的形成和扩大，因此可以推断，如果当地人继续沿袭他们的老传统，会导致沙漠继续扩大。故答案为A。
2、答案解析：答案为D。本题为推理题。考生需看清题意，要求选择与原文符合的事实。由第二段最后一句话 “At that time, huge river systems and many oases supported growing communities where a large number of people lived.”可知，答案为D。
由第一段 “Except for an occasional oasis, this vast land is mainly made up of sand, stones, and worn out mountains.”可知，A选项错误;B选项未提及;C选项也不是事实。故答案为D。
3、答案解析：答案为C。本题为推理题。由最后两段可知，科学家一直在研究让沙漠变绿洲的办法，并且已经付诸实践，取得了一定的成效，故可推断，科学家满怀希望想使撒哈拉沙漠恢复生机。故答案为C。
4、答案解析：答案为D。本题为主旨大意题。本文揭示了撒哈拉沙漠形成的原因，重点是介绍科学家依靠科学技术试图让这片沙漠变成绿洲的方法。故答案为D。
