Documentation of a software package is normally produced for two purposes. One is to explain the (1) ofthe software and describe how to use them. This is known as (2) because it is designed to be read by the user of the software. The other purpose of documentation is to describe the software's internal composition so that the system can be maintained later in its life cycle. Documentation ofthis type is known as ( 3 )
空白（2）处填（）
A.systemdocu mentation
B.userdocu mentation
C.venderdocu mentation
D.productdocu mentation.