除有特殊规定外，绝密、机密和秘密级事项的保密期限般不应超过的年限分别是多少年？

阅读理解。

     Can you imagine a world without the Internet? It's amazing to think about it.

     Now, China has more than 68 million Internet users, according to the China Internet Network Information

Centre. This is the second highest number of users in the world after the United States. Today, 44% of

Chinese "netizens (网民)" are teenagers."They spend about 13 hours every week online," said Qian Hulin, an

Internet expert. 

     The main reasons why teenagers surf the Web are to search for information, to communicate with others

and to have fun. On the Internet, teenagers can find out almost anything. And surfing the Web can help

students with their homework and widen their knowledge.

     Li Dong, a teacher at No. 41 Middle School in Shijiazhuang, likes her students to use the Internet. "When

we talk in class, students who surf the Net usually know more background information than the others," she

said.

     In addition, people can use the Internet to write letters or stories and send emails. Many teens keep in

touch with their friends online. It is cheaper than phoning somebody far away and also much quicker.

1. Which country has the most Internet user?

A. China

B. England

C. America

D. Japan

2. How many hours do teenagers spend on the Internet every week?

A. About 44 hours

B. About 13 hours

C. About 41 hours

D. About 10 hours

3. Why do teenagers surf the Web?

A. To get some money

B. To search for information

C. To have fun

D. Both B and C

4. Why does the teacher from Shijiazhuang like her students to use the Internet?

A. Because she wants to play computer games with them.

B. Because she wants to have a talk with them.

C. Because she wants to know what they do at home.

D. Because she wants to let them know more background information.

5. Which way is the cheapest and quickest to tell something to your friend who is far away?

A. Posting a letter.

B. Sending an e-mail.

C. Making a telephone call.

D. Going to visit your friend.

若二次函数y=ax²+c(a ≠0)，当x分别取x₁，x₂(x₁≠x₂)时，函数值相等，则当x取x₁+x₂时，函数值为 [ ]

A.  a+c

B.  a-c

C.  -c

D.  c

（），是来中国最早的苏联学前教育专家。

肾性急性肾衰竭最常见的病因是（）

A.链球菌感染

B.血容量减少

C.磺胺药过敏

D.肾缺血或肾毒性物质

E.急性尿路梗阻

可在已打开的Microsoft Excel和Word之间切换操作的是______。

A．Alt+F1

B．Ctrl+Alt

C．Alt+Tab

D．Ctrl+Shift