An internet is a combination of networks connected by (1).When a datagram goes from a source to a(2)，it will probably pass many (3)until it reaches the router attached to the destination network. A router receives a (4)from a network and passes it to another network. A router is usually attached to several networks. When it receives a packet, to which network should it pass the packet? The decision is based on optimization: Which of the available (5)is the optimum pathway?

空白（4）处应选择（）

A.destination

B.resource

C.packet

D.source

在开关仪表二阀组时，对两个阀的操作没有特殊要求，只要小心就可以了。