Passage 4 Questions 16 to 20 are based on the following passage. 　The age years of an individual is marked by evaluating one's values, experiencing a shift in outlooks, and a tendency to act rebelliously. It can also be a time when someone becomes excessively vulnerable to negative influences and inclined towards erous situations. On the other hand, for parents the period of their children's adolescence means regular worrying about their safety and formation as a citizen. Thus, a method of ensuring agers' security is needed, and curfews are often seen as such a measure, since they have proved their efficiency. At the same time, certain peculiarities exist about establishing curfews for children. 　The first and foremost reason for establishing curfews is children's security.Typical curfews require that agers under 17 years stay out of streets starting from 11pm or midnight. This is believed to protect them from crimes committed after nightfall, as well as from violating the law, and there exists serious evidence in favor of this conviction. For example, when New Orleans enabled a dusk-till-dawn curfew in 1994, the rates of juvenile （青少年的） crime were reported to fall more than 20 percent. Even more impressive results were recorded in Dallas, which reported a 30 percent decrease in violent juvenile crime, and a 21 percent decrease in the overall rates of crimes committed by young people. 　On the other hand, curfews can be seen as a prive measure which infringes on （诬害） the rights of young people, limiting their . This thesis is particularly supported by the fact that curfew violations and the respective charges are among the most often committed juvenile crimes in the United States. In addition, there were reports claiming that police arrested more non-white agers for curfew violations. All this can cause a ager to believe that they have crossed a psychological line dividing them as criminals; thus, such agers may start to see themselves as outlaws, which can contribute to committing more serious crimes than a curfew offense. 　What is important for a parent to remember when establishing a curfew for their kids is that a ager's misjudged perception of certain rules may cause them to react differently, or misbehave in some other way; this is proved by research conducted by the University of Minnesota, according to which s tend to react angrily against what they see as unfair. Considering this, parents should avoid the authoritarian（独断专行的） style of establishing curfews; instead, they should have a conversation with their ager that would be aimed at finding best conditions for a curfew that would satisfy both sides. Also, flexible curfews with s’ interests in mind tend to be more effective than rigid agreements.What does the word "curfew' mean in the passage?