The presentation layer is concerned with the （） and semantics of the informationexchanged between two systems． The processes in two systems are usually exchanginginformation in the form. of character（）,numbers, and so on． The information should be changed to bit（）before ．being transmitted． Because different computers use different encoding systems, the presentation layer is responsible for（）between these different encoding methods． The presentation layer at the sender changes the information from its sender-dependent format into a common format． The presentation layer at the receiving machine changes the common format into its（）dependent format．

The presentation layer at the receiving machine changes the common format into its（）dependent format．

A．receiver

B．user

C．server

D．customer

慢性脓胸患者的体征包括膈肌上升、肋间隙变宽和纵隔向健侧移位。