●Thenetworklayerprovidesservicestothetransportlayer.Itcanbebasedoneither(71).Inbothcases,itsmainjobis(72)packetsfromthesourcetothedestination.Innetworklayer,subnetscaneasilybecomecongested，increasingthedelayand(73)forpackets.Networkdesignersattempttoavoidcongestionbyproperdesign.Techniquesinclude(74)policy,caching,flowcontrol,andmore.Thenextstepbeyondjustdealingwithcongestionistoactuallytrytoachieveapromisedqualityofservice.Themethodsthatcanbeusedforthisincludebufferingattheclient,trafficshaping,resource(请作答此空)，andadmissioncontrol.Approachesthathavebeendesignedforgoodqualityofserviceincludeintegratedservices(includingRSVP),differentiatedservices，andMPLS.
A.distribution B.guarantee C.scheme D.reservation正确答案D
