OSI is a theoretical model that shows how any two different systems can communicate with each other．Router, as a networking device, operate at the (71) layer of the OSI model．

A．transport

B．application

C．network

D．physical

物流信息的分类、研究和筛选等工作的难度比较大，这是由于物流信息的信息量大、分布广、种类多这些特点决定的。