[简答题] About three hundred years ago, there were approximately half a billion people in the world. In the two centuries that followed the population doubled, and, by 1850, there were more than a billion people in the world. It took only 75 years for the figure to double once more, so that now the population figure stands at approximately six and one half billion Each day the population of the world increases by about 150,000. In former centuries the population grew slowly. Famines, wars, and epidemics, such as the plague and cholera, killed many people. Today, although the birth rate has not changed significantly, the death rate has been lowered considerably by various kinds of progress. Machinery has made it possible to produce more and more food in vast areas, such as the plains of America and Russia Crops have increased almost everywhere and people are growing more and more food. New forms of food preservation have also been developed so that food need not be eaten as soon as it has grown. Meat, fish, fruit and vegetables can be dried, tinned or frozen, then stored for later use. Improvement in communications and transportation has made it possible to send more food from the place where it is produced to other places where it is needed. This has helped reduce the number of famines. Generally speaking, people live in conditions of greater security. Practices such as the slave trade, which caused many useless deaths, have been stopped. Progress in medicine and hygiene has made it possible for people to live longer. People in Europe and North America live, on the average, twice as long as they did a hundred years ago. In other countries, too, people generally live much longer than they once did. Babies, especially, have a far better chance of growing up because of increased protection against infant disease. However, all countries do not benefit to the same degree from this program in medicine and hygiene.a particular amount expressed as a number （Para. 1）