Many of the world's pollution problems have been caused by the crowding of large groups of people into the cities. Supply for the needs of the people leads to further pollution by industry. If the rapid increase of world population in countries continues at the present rate, there may be much greater harm to human beings. Some scientists speak of the increase in numbers of people as "population pollution." About 2,000 years ago, the world population was probably about 250 million. It reached a billion in 1850. By 1930 the population was two billion. It is now three and a half billion. It is expected to double by the year 2,000. If the population continues to grow at the same rate, there will be 25 billion in the world a hundred years from now. Man has been using the earth's resources more and more rapidly over the past years. Some of them are almost used up. Now many people believe that man's greatest problem is how to control the growth of the population. The material supplies in the world will be far from enough to support the human population in time to come, if the present rate of increase continues. Already there is overcrowding in many cities and starvation in some countries. Should man's population keep on growing so rapidly as before? Many people believe that human survival in the future depends on the answer to this question. The rapid increase in population is considered to be    _   .
the increase of resources population pollution the development of mankind the present rate