In 1971 there were about 3,700 million people in the world. If the population were 【C1】______ evenly over the earth' s surface there would be about 50 people to the square mile; but there are vast areas of desert and mountain and tropical forest 【C2】______ are uninhabited, 【C3】______ at the other 【C4】______ , in the great cities millions may live within a few square miles. 【C5】______ of the world' s population is concentrated on only a small 【C6】______ of the earth' s land surface, in the rich valleys and 【C7】______ plains, because people up to the present time have 【C8】______ to congregate in place where the climate and soil make it easy to grow food and obtain shelter. A 【C9】______ world population and the discoveries of science 【C10】______ this pattern of distribution in the future. As men slowly learn to master diseases, control floods, prevent famines, and stop wars, fewer people die every year; and in 【C11】______ the population of the world is steadily 【C12】______ . When numbers 【C13】______ , the extra mouths must be fed. New lands must be brought 【C14】______ cultivation, or land already 【C15】______ , made to yield larger crops. In some areas the accessible land is largely so intensively cultivated 【C16】______ it will be difficult to make it 【C17】______ more food. in some areas the population is so dense that the land is divided into. units 【C18】______ tiny to allow for much improvement in farming methods. 【C19】______ a large part of this farming population drawn 【C20】______ into industrial occupations, the land might be farmed much more productively by modern methods. 【C1】______