题目内容：
Prolonging Human LifeProlonging human life has increased the size of the human population．Many people alive today would have died of childhood diseases if they had been born 100 years ago．Because more people live longer, there are more people around at any given time．In fact, it is a decrease in death rates, not an increase in birthrates that has led to the population explosion.
Prolonging human life has also increased the dependency load．In all societies, people who are disabled or too young or too old to work are dependent on the rest of society to provide for them．In hunting and gathering cultures, old people who failed to follow the others were left behind to die．In times of famine, infants might be allowed to die because they couldn't survive if their parents starved, whereas if the parents survived they could have another child．In most contemporary societies, people feel a moral obligation to keep people alive whether they can work or not．We have a great many people today who live past the age at which they want to work or are able to work; we also have rules that require people to retire at a certain age．Unless these people were able to save money for their retirement, somebody else must support them．In the United States many retired
people live on social security checks that are so little that they must live in near poverty.Older people have more illness than young or middle-aged people; unless they have wealth or private or government insurance, they must often "go on welfare" if they have a serious illness.
When older people become senile or too weak and ill to care for themselves, they create grave problems for their families．In the past and in some traditional cultures, they would be cared for at home until they died．Today, with most members of a household working or in school, .there; is often no one at home who can :care for a sick or weak person．To, meet this need, a great many nursing homes and convalescent hospitals (康复医院) have been built．These are often profit-making organizations, although some are sponsored by religious and other nonprofit groups．While a few of these institutions are good,most of them are simply "dumping grounds" for the dying in which "care" is given by poorly paid, overworked, and under skilled personnel.The writer believes that the population explosion results from____. A．an increase in birthrate
B．the industrial developmen
C．a decrease in death rate
D．cultural advance
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