The network layer provides services to the transport layer．It can be based on either (67) ．In both cases，its main job is (68) packets from the source to the destination．

In network layer，subnets can easily become congested，increasing the delay and (69) for packets．Network designers attempt to avoid congestion by proper design．Techniques include (70) policy,caching，flow control，and more．

The next step beyond just dealing with congestion is to actually try to achieve a promised quality of service．The methods that can be used for this include buffering at the client，trafficshaping，resource (71) ，and admission contr01．Approaches that have been designed forgood quality of service include integrated services(including RSVP)，differentiated services，andMPLS．

68（）

A．sending

B．receiving

C．switching

D．routing

关于人和高等动物激素的叙述，错误的是（）

A.含量极微但能显著调节生命活动

B.是信息分子，不直接参与细胞代谢

C.一经靶细胞接受并起作用后就被灭活了

D.不同激素之间都是相互促进的关系