Man can go on increasing his numbers at the present rate. In the next 30 years man will face a period of crisis. Some experts believe that there will be a widespread food shortage. Other experts think that this is too pessimistic, and that man can prevent things from getting worse than they are now. But remember that two-thirds of the people in the world are undernourished or starving now.
One thing that man can do is to limit the number of babies born. The need for this is obvious, but it is not easy to achieve. People have to be persuaded to limit their families. In the countries of the population explosion, many people like big families. The parents think that this brings a bigger income for the family and ensures there will be someone in the family who will look after them in old age.
Several governments have adopted birth control policies in recent years. Among them are Japan, China, India and Egypt. In some cases the results have not been successful. Japan has been an exception. The Japanese adopted a birth control policy in 1948. People were encouraged to limit their families. The birth rate fell from 34.3 per thousand per year to about 17 per year at present.
It is difficult to carry out birth control because ____.（ ）
文章第二段最后两句讲许多人喜欢大家庭。父母们认为这样可以增加收入，而且老了之后会有人给他们养老。
