单选题 Statistically, each of these new changes in law-enforcement has made some difference to the picture. Yet it seems probably that the factors that have really brought the crime rates down have little to do with policemen or politicians, and more to do with cycles that are beyond their control.
The first of these is demographic. The fall in the crime rate has coincided with fall in the number of young men between the ages of 15 and 21, the peak age for criminal activity in any society, including America. In the same way, the rise in the crime rate that started in the early 1960s coincided with the teenage years of the baby-boomers. As the boomer generation matured, married, found jobs and shoulder mortgages, so the crime rate fell.
This encouraging trend was quickly overshadowed, starting in the mid-1980s, by a new swarm of teenagers caught up in a new sort of depravity: the craze for crack cocaine. Crack brought with it much higher levels of violence and, in particular, soaring rates of handgun murders by people less than 25 years old. Yet the terror became too much, and the young began to leave crack alone. Within a few years, at least in most big cities, the drug market had stabilized and settled, even moving indoors; the tuff-wars were over, and crack itself had become passe. Studies of Brooklyn by Richard Curtis, of the John Jay College of Criminal Justice, show the clear connection; around 1992, many young bloods decided to drop the dangerous life of the street in favor of steady jobs. In direct consequence, the local crime rate fell.
Murder rates among Americans older than 25 had already been declining since 1980. Here, according to Alfred Blumstein, a professor of criminology at Carnegie-Mellon University, there may be even longer term social factors involved. In an age of easy divorce and more casual relationships, men and women are less likely to murder their partners: between 1976 and 1996, such murders fell by 40%. The decline in alcohol consumption, too, means that fewer bar-room brawls leave a litter of corpses on a Friday night.
It seems that changing social trends also sometimes lie behind the fall in property crime. Burglars tend not to steal television sets now because almost everyone has one; their value on the street has plummeted, At the same time, the fact that people stay in watching their sets, rather than going out, deters would-be burglars. Extra garages are standard in the suburbs, to safeguard extra cars; credit cards mean that shoppers carry less cash in their pockets; people working from home, by means of computers, can keep a closer watch over their streets.
Lastly, people are going to greater lengths to protect themselves and their property than they did in the past. This is partly because of the huge fear of crime that preceded the present decline, and partly because even with recent increases in the number of policemen--the ratio of police to violent crimes reported is still way below what it was in the 1960s.

单选题 This passage mainly concerns about ______.
A. the factors influencing the crime rate.
B. the demographic causing the fall of crime rate.
C. murder rate becoming lower.
D. the effort of people to fight against crime.

单选题 In early 1970s the crime rate was ______.
A. the same as that in early 1960s.
B. lower than that in early 1960s.
C. higher than that in mid-1980s.
D. the same as that in mid-1980s.

单选题 Murder rate among Americans older than 25 declined because ______.
A. they married and found jobs.
B. they had to shoulder mortgage.
C. they were in an age of easy divorce.
D. they made great effort to protect themselves.

单选题 The word plummet in the 2nd line of the 5th paragraph means ______.
A. drop.
B. disappear.
C. enhance.
D. stabilize.

单选题 Why do people make greater efforts to protect themselves?
A. Because they fear the crime preceding the present decline.
B. Because the policeman has become fewer.
C. Because they were taught to do so.
D. Because their extra garage are standard in the suburb.