Documentation of a software package is normally produced for two purposes.One is to explain the(1)of the 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 intemal composition so that thesystem can be maintained later in its life cycle.Documentation of this type is known as(3)

空白（3）为（）

A.system documentation

B.user documentation

C.vender documentation

D.license documentation

引起信号出现严重失真的是（）。

A.满调幅

B.过调幅

C.欠调幅