关于作用力和反作用力，下列说法中正确的是（   ）

A．作用力和反作用力可能不是同种性质的力；

B．地球对人的作用力和人对地球的作用力是大小相等且方向相反的；

C．一个力的作用力和它的反作用力的合力为零；

D．汽车牵引拖车前进，当车加速时，汽车对拖车的作用力大于拖车对汽车的作用力；

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．database

B．data representation

C．protocols

D．data communication