问题 单项选择题

下列网络服务中, (26) 是文件传输协议,默认端口号 (27)

A.21
B.25
C.80
D.23

答案

参考答案:A

解析:FTP(文件传输协议)是一个用于从一台主机到另一台机传送文件的协议。FTP的主要作用就是让用户连接到一个远程计算机(这些计算机上运行着FTP服务器程序)查看远程计算机有哪些文件,然后把文件从远程计算机上复制到本地计算机,或把本地计算机的文件送到远程计算机去。
FTP要用到两个TCP连接,一个是命令链路,用来在FTP客户端与服务器之间传递命令;另一个是数据链路,用来上传或下载数据。FTP有两种工作方式:PORT方式和PASV方式,它们的中文含义分别为主动式和被动式。
PORT(主动)方式的连接过程是:客户端向服务器的FTP端口(默认是21)发送连接请求,服务器接受连接,建立一条命令链路。当需要传送数据时,客户端在命令链路上用PORT命令告诉服务器:“我打开了××端口,你过来连接我”,于是服务器从20端口向客户端的××端口发送连接请求,建立一条数据链路来传送数据。
PASV(被动)方式的连接过程是:客户端向服务器的FTP端口(默认是21)发送连接请求,服务器接受连接,建立一条命令链路。当需要传送数据时,服务器在命令链路上用PASV命令告诉客户端:“我打开了××端口,你过来连接我”,于是客户端向服务器的××端口发送连接请求,建立一条数据链路来传送数据。

完形填空

The word “plastic” comes from the Greek word “Platicos” and is used to describe_1_which can be easily shaped.

The history of plastics is longer than you might_2_.In fact _3_ manmade plastics ever to appear on the market was made over a hundred years ago. It was called “celluloid(赛璐璐)”.It was discovered by both an Englishman and an American in the same year._4_it was the Americans who first produced it on a large scale during the year 1860.Everybody was _5_by this new material which could be moulded (用模子做) into shapes and _6_so cheap to buy.

Poor young men _7_in cities with lots of smoke and dirt were_8_ to buy white celluloid collars(领子).The collars were hard and uncomfortable. But they did not have to be sent to wash. The poor young men _9_ rub them clean every evening with soap and water! Poor mothers,

_10_ had not been able to afford playthings _11_their children, were now able to buy them playthings _12_ of celluloid. But celluloid had one very serious fault. It caught fire very easily. In fact it burned even more quickly than wood or cloth. There were many terrible accidents, particularly _13_ children. For years scientists worked hard to find a better plastic material than celluloid. They had _14_ success. Then in 1932 an American scientist called Baekeland produced a hard plastic material which did not burn. _15_ became known _16_ bakelite(酚醛塑料).Other plastic materials like bakelite were _17_ produced. They _18_ to make electrical fittings and plates and cups. Poor people liked them because they were both cheap and safe, but rich people _19_ them because they were cheap, and because they could only be made _20_dark colours.

1. A. something    B. everything  C. thing            D. anything

2. A. wish        B. think       C. want            D. hope

3. A. first         B. at first      C. for the first      D. the first

4. A. So          B. But        C. And            D. As

5. A. surprised     B. excited     C. told            D. frightened

6. A. be          B. were       C. was             D. is

7. A. waiting      B. running    C. walking          D. working

8. A. going      B. sure        C. about            D. able

9. A. could       B. ought to     C. must            D. should

10. A. which      B. who       C. whom            D. that

11. A. for        B. like        C. in               D. of

12. A. done       B. produced   C. worked           D. made

13. A. between    B. on        C. among            D. about

14. A. much      B. great.      C. never             D. little

15. A. The man   B. He         C. It                D. The scientist

16. A. as        B. with        C. by               D. to

17. A. already    B. still         C. also             D. yet

18. A. used      B. hoped       C. were used         D. had

19. A. took     B. loved       C. enjoyed           D. disliked

20. A. in       B. by          C. from            D. of

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