Both bus and tree topologies are characterized by the use of multipoint (16) ．For the bus，all stations attach，through appropriate hardware (17) known as a tap，directly to a linear transmission medium，or bus．Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the (18) ．A transmission from any station propagates the length of the medium in both directions and can be received by all other (19) ．At each end of the bus is a (20) ，which absorbs any signal，removing it from the bus．

A．routers

B．stations

C．servers

D．switches

密码分析的目的是什么？（）

A、确定加密算法的强度

B、增加加密算法的代替功能

C、减少加密算法的换为功能

D、确定所使用的换位