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问题 单项选择题

在下列叙述中，正确的一条是( )。

A．在CRT显示器中，字符终端只能显示字符，而图形终端只能显示图形

B．在RS-232C标准中规定使用DB-25连接器，它的25个引脚都已作了明确的定义

C．从显示器的工作原理上讲，CRT显示器采用隔行扫描，比通行扫描可以获得更高的分辨率

D．RS-232C接口使用的也是数字逻辑信号，它可以与计算机的各种输入输出信号直接连接

答案

参考答案：C

解析：通行扫描：电子束采用光栅扫描方式，从屏幕左上角一点开始，向右逐点进行扫描：到达最右端后，又回到下一条水平线的左端。而隔行扫描指电子束在扫描时每隔一行扫一线，完成一屏后再返回来扫描剩下的线。采用隔行扫描比通行扫描可以获得更高的分辨率。

单项选择题

对于现场使用的塔式起重机及有特殊安全要求的设备，进入现场后在使用前，必须经( )鉴定，符合要求并办好相关手续后方可允许承包单位投入使用。

A．监理单位

B．当地建设行政主管部门

C．承包单位

D．当地劳动安全部门
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单项选择题

Typically, these are concern with the establishment of (66) the network and with the control of the flow of messages across this channel. The provision of such facilities is only part of the network requirements, however, since in many applications the communicating computers may be of different types. This means that they may use different programming languages and, more importantly, different forms of (67) interface between user(application)program, normally referred to, as application processes or APs, and the underlying communication services may be (68) For example, one computer may be a small single -user computer, while another may be a large (69) system. In the earlier days of computer communication, these issues meantthat only closed communities of computers ( that is, from the same manufacturer) could communicate with each other in a meaningful way. IBM’s Systems Network.
Architecture (SNA) and DEC’s Digital Network Architecture (DNA) are just two examples of Communication software packages produced by manufacturers to allow their systems to be int erconnected together. These proprietary packages, however, of which there am still many inexistence, do not address the problem of universal interconnect ability, or open systems interconnection. In an attempt to alleviate this problem, (70) , in the late 1970s, formulated a reference model to provide a common basis ( or the coordination of standards developments and to allow existing and evolving standards activities to beplaced into perspective with one another.

A. communication channel access B. protocols
C. data channel D. public communication
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