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根据下列材料，回答问题。Text 2
In the 21 st century there's no doubt that frightening new infectious diseases will appear.Today new viruses are coming out of nature and "discovering" the human species.Just since 1994, at least 30 new viruses have appeared.
Viruses are moving into the human species because there are more of us all the time.From a virus' point of view, we look like a free lunch that's getting bigger.In nature viral diseases tend to break out when populations increase rapidly and become densely packed.Then many deaths occur and the population drops.This is nature's population-control mechanism.There is no reason to think the human race is free from the laws of nature.
Giving these laws an extra push will be the rise of megacities--huge densely packed cities in less developed nations.A United Nations study predicts that by the year 2015, there will be 26 ex-tremely big cities on the planet.By then, some megacities could have 30 million or more people.That is approximately the total population of California.Imagine all the people in California crowded together tightly into one vast city.Then remove most doctors and medical care, take away basic sanitation and hygiene, and you have a biological "time bomb".Now make eight or ten such "bombs" and plant them around the world.
Also consider the biological weapons the world will be capable of producing in the future.The 20th century saw the creation of great and terrible weapons based on the principles of nuclear physics.The 21st century will see great and terrible weapons based on the knowledge of DNA and the genetic code.As biotechnology becomes more sophisticated and powerful, biologists will learn how to mix genes of different microbes to create unnatural strains that can be turned into deadly, effective weapons.
Biological weapons are a disgrace to biology.Most biologists haven't wanted to talk or even think about them.The physicists lost their innocence when the first nuclear bomb went off in 1945.The biologists will lose their innocence when the first biological weapon spreads through the human species.
Yet the 20th century survived despite the existence of the nuclear bomb.There was great economic and scientific progress and much human happiness.The same can be true in the next century.We may not completely win the 21st century microbe war, but I am confident that we won't lose it.
Infectious diseases are more likely to occur where A．different human races are mixed in living.
B．many people live in crowded communities.
C．population drops due to nature's control mechanism.
D．new viruses appear for the first time.
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