An internet is a combination of networks connected by  （）  .When a datagram goes from a source to a  （） ，it will probably pass many  （）  until it reaches the router attached to the destination network．A router receives a  （）  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  （）  is the optimum pathway.

When a datagram goes from a source to a （）

A．user

B．host

C．city

D．destination