In an early survey conducted in 1888, a billion and a half people inhabited the earth. Now, the population is about six billion and is growing fast. Even though the rate of growth has begun to slow down, most people believe the population size may be eight billion during the next several years. If we examine the amount of land available for this ever increasing population, we begin to see the problem. Not all land is useful to us because it cannot produce food. We can cut out about one fifth of it because it is covered by snow and ice. Then we can cut out another fifth because it is desert. Another fifth is mountains. The other fifth does not have enough soil for crops to grow because it is bare rock. Obviously, with so little land to support us, we should take great care not to reduce it any more or with the rapidly growing population we may sooner or later lack food to eat and place to live in.31.What is the main topic of the passage?A. The world's population and the shortage of cultivated land. B. The growth of the world's population. C. The necessity of reducing the rate of birth. D. The shortage of cultivated land.