Networks can be interconnected by different devices．In the physical layer，networks can be connected by (64) or Hubs, which just move the bits from one network to an identical net-work. One layer up we find bridgas and swiehes, which operate at data link layer. They can accept (65) , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two netwoks. If two networks have (66) network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between the two transport connections. Finally, in the application layer, application gateways translate message

(67) . As an example, gateways between Internet e-mail and X.400 e-mail must (68) the e-mail message and change various header fields.

67（）

A．syntax

B．semantics

C．languages

D．format

患者，男性，58岁，突感剧烈头痛、恶心、呕吐；查脑膜刺激征阳性，脑脊液检查呈血性，压力增高。最可能的诊断是（）

A.脑血栓形成

B.脑栓塞

C.高血压脑病

D.蛛网膜下腔出血

E.脑供血不足