数据库系统在其内部具有3级模式，用来描述数据库中全体数据的全局逻辑结构和特性的是（）

（a²+b²）²-4a²b²=______．

下面A、B、C、D、E、F是实验室制取、收集气体及氧气的化学性质实验装置图，请读题并根据各图回答以下问题：

（1）实验室中常用高锰酸钾加热分解制取氧气。请回答应该选用______装置（填字母）。写出化学反应文字表达式：_________________，该反应属于______反应。

（2）被称为“绿色氧化剂”的过氧化氢俗称双氧水，是一种无色液体。其溶液在二氧化锰做催化剂的条件下能迅速分解，生成氧气。如果改用双氧水制取氧气，请回答：　　

①通过长颈漏斗应加入的物质是         ；锥形瓶中应放入的物质是        （填化学式）。

②写出利用该方法制取氧气反应的文字表达式：                 ；要想收集一瓶较为纯净的氧气，最好选择装置               （填字母）。

③请简述该方法与用高锰酸钾制取氧气相比优点有哪些。（至少写出两点）

                        ；                    。

（3）小路同学用过氧化氢制取并收集了一瓶氧气，准备做铁丝燃烧的实验，如图F所示，他在瓶底加了少量水。请你来想一想，加水的目的是               。

（4）实验室常用硫化亚铁(FeS)固体与稀硫酸(H₂SO₄)在常温下反应制硫化氢气体和硫酸亚铁(FeSO₄)。硫化氢气体是无色有刺激性气味的气体，它的密度比空气大，可溶于水。

①试写出实验室制取硫化氢气体的化学反应的文字表达式：_________________。

②制取硫化氢时，发生装置应选用上图所示的________装置（填字母，下同）；收集硫化氢气体时，只能选用__________装置。

城市轨道交通工程中地铁车站应满足在发生事故灾害时，能在( )min内将一列车乘客，站台上候车人及车站工作人员全部撤离开站台。

A．5

B．6

C．7

D．8

Radiation occurs from three natural sources: radioactive material in the environment, such as in soil, rock, or building materials; cosmic rays; and substances in the human body, such as radioactive potassium in bone and radioactive carbon in tissues. These natural sources account for an exposure of about 100 millirems a year for the average American.
The largest single source of man-made radiation in medical x-rays, yet most scientists agree that hazards from this source are not as great as those from weapons-test fallout, since strontium-90 and carbon-14 become incorporated into the body, hence delivering radiation for an entire lifetime. (46) The issue is, however, by no means uncontroversial; indeed, the last two decades have witnessed intensified examination and dispute about the effects of low-level radiation.
A survey conducted in Britain confirmed that an abnormally high percentage of patients suffering from arthritis of the spine who had been treated with x-rays contracted cancer. Another study revealed a high incidence of childhood cancer in cases where the mother had been given x-rays. (47) These studies have pointed to the need to re-examine the assumption that exposure to low linear energy transfer presented only a minor risk.
Recently, examination of the death certificates of former employees of a West Coast plant which produces plutonium for nuclear weapons revealed markedly higher rates for cancers of the pancreas, lung, bone marrow and lymph systems than would have been expected in a normal population.
(48) While the National Academy of Sciences committee attributes these differences to chemical or other environmental causes, rather than radiation, other scientists maintain that any radiation exposure, no matter how small, leads to an increase in cancer risk. (49) It is believed by some that a dose of one rem, if sustained over many generations, would lead to an increase of one percent in the number of 1,000 disorders per million births.
In the meantime, regulatory efforts have been disorganized, fragmented, and inconsistent, characterized by internecine strife and bureaucratic delays. A Senate freport concluded that coordination of regulation among involved departments and agencies was not possible because of jurisdictional disputes and confusion. (50) One Federal agency has .been unsuccessful in its efforts to obtain sufficient funding and manpower for the enforcement of existing radiation laws, and the chairperson of a panel especially created to develop a coordinated Federal program has resigned.

本题用复选框来控制字体的显示，窗口中有一个标签和两个复选按钮，这两个复选按钮分别对应的字体的两个特点：加粗和倾斜，任意选中一个按钮或两个都选中，标签上的字符串就显示对应的字体。
import java. awt. * ;
import java. awt. event. * ;
import javax. swing. * ;
class CheckBoxFrame extends JFrame implements ActionListener
public CheckBoxFrame()
setTitle("CheckBoxTest");
setSize(300,200);
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
System. exit(0);

);
JPanel P=new JPanel();
bold=addCheckBox(P,"Bold");
italic=addCheckB0x(P,"Italic");
getContentPane(). add(P."South");
panel=new CheckBoxTestPanel();
getContentPane(). add(panel,"Center");

public JCheckBox addCbeckBox(JPanel p, String name)
JCheckBox c=new JCheckBox(name);
C. addActionListener(______);
P. add(c);
return C;

public void______
int m=(bold. isSelected() Font. BOLD:0)+(italic. isSelected() Font. ITALIC:0);
panel. setFont(m);

private CheckBoxTestPanel panel;
private JCheckBox bold;
private JCheckBox italic;

class CheckBoxTestPanel extends JPanel
public CheckBoxTestPanel()
setFont(Font. PLAIN);

public void setFont(int m)
setFont(new Font("SansSerif", m, 12));
repaint();

public void paintComponent(Graphics g)
super. paintComponent(g);
g. drawstring("The quick brown fox jumps overthe lazy dog.", 0,50);


public class java2
public static void main(String[] args)
JFrame frame=new CheckBoxFrame();
frame. show();