题目内容：
根据以下短文，回答题。第一篇 Prolonging Human Life
Prolonging 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 birth rates, that has led to the population explosion.
Prolonging human life has also increased the dependency (依赖) 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 could not keep up might be left behind to die. In times of famine (饥荒., infants might be allowed to die because they could not 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 which require people to retire at a certain age. Unless these people were able to save money for their retirement, somebody else must support them. I the United States many retired people love on social security checks which 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 (年老的) 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 institute ones are good, most of them are simply dumping grounds for the dying in which care is given by poorly paid, overworked and underskilled personnel.
The writer believes that the population explosion results from
A an increase in birthrates.
B the industrial development
C a decrease in death rates.
D cultural advances.
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