Documentation of a software package is normally produced for two purposes．One is toexplain the (71) ofthe software and describe how to use them．This is known as (72) because it is designed to be read by the user of the software．The other purpose ofdocumentation is to describe the software’s internal composition SO that the system can bemaintained later in its life cycle．Documentation of this type is known as (73) ．

A．system documentation

B．user documentation

C．vender documentation

D．1icense documentation

国家规范规定，GPS测量控制可分为AA、A、B、C、D、E六个等级，其中（）级可作为建立地心参考框架的基础。

A.AA

B.A

C.B

D.C