题目内容：
根据材料回答题：第二篇Prolonging Human Litre
Prolonging human life has increased the size of the human population. Many people alive todaywould have died of childhood diseases if they had been bdrn 100 years ago. Because more people livelonger, there are more people around at any given time. In fact, it is a decrease in death rates, not anincrease in birthrates, that has led to the population explosion.
Prolonging human life has also increased the dependency (依赖) load. In all societies, people whoare 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. Intimes of famine (饥荒), infants might be allowed to die because they could not survive if their parentsstarved (使挨饿), whereas if the parents survived they could have another child. In most contempora-ry societies, people feel a moral obligation to keep people alive whether they can work or not. We havea great many people today who live past the age at which they want to work or are able to work; wealso have rules which require people to retire at a certain age. Unless these people were able to savemoney for their retirement, somebody else must support them. In the United States, many retired peo-ple live on social security checks which are so little that they must live in near poverty. Older peoplehave more illness than young or middle-aged (中年) people; unless they have wealth or private or gov-ernment insurance, they must often "go on welfare (福利 )" if they have a serious illness.
When older people become senile or too weak and ill to care for themselves, they create graveproblems for their families. In the past and in some traditional cultures, they would be cared for athome until they died. Today, with most members of a household working or in school, there is oftenno one at home who can care for a sick or weak person. To meet this need, a great many nursing homesand convalescent hospitals have been built. These are often profit-making organizations, althoughsome 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, o-verworked, and under skilled 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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