图是一种水位自动报警器的原理图。当水位到达金属块A时，控制电路接通，这时电磁铁有_____________而吸引衔铁，灯泡_____________发光（选填“L₁”或“L₂”）

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（5）处应选择（）

A.pathways

B.routers

C.diagrams

D.calls