21（）

A．As
B．Like
C．Since
D．Because

A recent development is the local area network (LAN). (21) its name implies, it serves a local area-possibly as small as a single room, typically an area like an university campus or the premises of a particular business. Local area networks were developed to (22) a need specific to microcomputers the sharing of expensive resources. Microcomputers are cheap, but highcapacity disc stores, fast and/or good quality printers, etc. are expensive. The object of the LAN is to allow (23) microcomputers shared access to these expensive resources, since the microcomputers are cheap, it is a necessary feature of a LAN that the method of connection to the network, and the network hardware (24) , must also be cheap.

A local area network links a number of computers and a number of servers which provide communal facilities, e. g. file storage. (A server usually includes a small microprocessor for control purposes. ) The computers and servers are known as stations. There are two methods of (25) in common use, rings and broadcast networks.

In the ring method( often called a Cambridge Ring) all the stations are linked in a ring, (26) includes one special station, the monitor station.

In broadcast networks, all the stations are (27) to a single linear cable(usually coax cable), and any transmission will be received by all stations.

(28) technology is used, local area networks are a development of the greatest importance. (29) as programming is simplified by an approach that thinks in terms of small procedures or programs, each doing a well-defined job, the computer system of tomorrow is likely to be (30) lots of small systems, each doing a specific job, linked by a local area network.