The presentation layer is concerned with the (1) and semantics of the information exchanged between two systems. The processes in two systems are usually exchanging information in the form of character(2), numbers, and so on. The information should be changed to bit (3) before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for(4) 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(5)-dependent format.

空白（2）处应选择（）

A.numbers

B.functions

C.strings

D.codes

碳酸盐水垢通常指碳酸盐质量分数大于（）的水垢。

A、20%

B、80%

C、50%

D、25%