题目内容
设E、E1、E2和E3是关系数表达式，下面列出的式子中，不满足等价变换规则的是
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设有关系R=(A，B，C)，和关系代数表达式：
Ⅰ．πA,C(σB=5(R))
Ⅱ．σB=5(πA,C(R))
Ⅲ．σπA,C(πB=5(R))
上述关系代数表达式中，与SQL语句selectdistinct A，C from R where B=5等价的是
在窗体上画一个名称为Labell、标题为“ViSBalBasic考试”的标签，两个名称分别为 Command1和Command2、标题分别为“开始”和“停止”的命令按钮，然后画一个名称为Timer1的计时器控件，并把其Interval属性设置为500，如图所示。
编写如下程序： Private Sub Form Load() Timerl.Enabled = False End Sub Private Sub Command1 Click() Timer1.Enabled = True End Sub Private Sub Command2 Click() Timerl.Enabled = False End Sub Private Sub Timerl Timer() If Labell．Left＜Width Then Labell.Left = Labell.Left+20 Else Labell.Left = 0 End If End Sub 程序运行后单击“开始”按钮，标签在窗体中移动。 对于这个程序，以下叙述中错误的是
The traditional two-parents family is fast giving way in the America of the 1980s to households in which one adult must juggle the often enormous demands of making a living and raising children.
For many, single parenthood is synonymous with economic need. More than 3 million single-parent families live in poverty, according to the Census Bureau, and joblessness, plus cuts in public assistance, has helped drive up the number of poor children in such families by about 20 percent in just three years.
The biggest burden falls on households that are headed by single mothers. Nearly half of these families are below the poverty as" the most compelling social fact "of the last 10 years.
This deprivation is not only hard on its victims but expensive for taxpayers since single women and their offspring receive 40 to 80 percent of the benefits in various welfare programs that cost the government a total of 40 billion dollars a year. Despite cuts in benefits averaging 10 percent, rising number of eligible women are likely to keep the overall cost up, according to economist Alice Rivlin, former director of the Congressional Budget office.
Fanning the single-parent spiral are two dramatic offshoots of the sexual revolution: divorce and unwed(未婚的) motherhood. The divorce rate has doubled in the last 15 years, and the number of illegitimate births has more than doubled to 700,000 annually. One tenth of white children and more than one half of black children are now born out of wedlock. What's more, there is a strong tendency now for women and teenagers who have illegitimate children to keep rather than put them up for adoption.
Typical is Rufina Nera of Los Angeles. When she became pregnant at 15, abortion was never mentioned in her home. Instead, her mother encouraged her to have the child, says Nera, adding: "She even gave a baby shower for me."
Now, Nera shares a crowded bedroom with her 2-year-old daughter as well as her sister. She hope no hope of help from the father, although he remarked during the only time he saw the child that she was prettier than his other illegitimate baby. Even so, Nera tries to keep her attention on two goals: moving into her own apartment and getting enough education to become a secretary or a nurse. Her first step along that path is attending Ramona High School, an "opportunity school" where she and 110 other girls study while their babies are cared for in a nursery.
What effect does joblessness and cuts in public assistance have on children of single - parent families？
Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
Acting is such an over-crowded profession that the only advice that should be given to a young person thinking of going on the stage is "Don't!" But it is useless to try to discourage someone who feels that he must act, though the chances of his becoming famous are slim. The normal way to begin is to go to a drama school. Usually only students who show promise and talent are accepted, and the course lasts two years. Then the young actor or actress takes up work with a repertory company, usually as an assistant stage manager. This means doing everything that there is to do in the theatre: painting scenery, looking after the furniture, taking care of the costumes, and even acting in very small parts. It is very hard work indeed. The hours are long and the salary is tiny. But young actors with the stage in their blood are happy, waiting for the chances of working with a better company, or perhaps in films or television.
Of course, some people have unusual chances which lead to fame and success without this long and dull training. Connie Pratt, for example, was just an ordinary girl working in a bicycle factory. A film producer happened to catch sight of her one morning waiting at a bus stop, as he drove past in his big car. He told the driver to stop, and he got out to speak to the girl. He asked her if she would like to go to the film studio to do a test, and at first she thought he was joking. Then she got angry and said she would call the police. It took the producer twenty minutes to tell Connie that he was serious. Then an appointment was made for her to go to the studio the next day. The test was successful. They gave her some necessary lessons and within a few weeks she was playing the leading part opposite one of the most famous actors of the day. Of course, she was given a more dramatic name, which is now world-famous. But chances like this happen once in a blue moon!
According to the passage, the main reason why young people should be discouraged from becoming actors is ______.
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