单选题 How many people can live on the face of the earth? No one knows the answer. It depends on how much food people can grow 1 destroying the environment.
More people now exist than ever before, and the population 2 growing. Every 15 seconds, about 100 babies are born. Before the end of this century, the earth may 3 10 billion people!
To feed everyone, farmers must grow more food. They are trying to do so. World food production has gradually 4 over the years. In some parts of the world, 5 , the population is growing faster than the food supply. Some experts fear the world will not be able to produce enough food for a 6 that never stops increasing.
To grow more crops on the same 7 of land, farmers use fertilizers and pesticides (杀虫剂). Some plant new kinds of grains that produce more food. These things help— 8 they don"t provide perfect solutions. The chemicals in fertilizers and pesticides can pollute water supplies. The new seeds developed by scientists have reached the 9 of what they can produce.
When hungry people can get no more out of 10 field, they clear trees from hills and forests for new farmland, and in doing so they expose the soil. Then rain and floods may strip the topsoil from fields. This process is called erosion. Each year erosion steals billions of tons of topsoil from farmers.