系统的功能是系统的行为中人们所（）的即设计中所要求实现且能完成（）的部分。
参考答案：需要；一定任务
_____ is the most difficult of _____. [ ]
A. Lesson Nine; Book Three
B. The lesson nine; the book three
C. The nine lesson; the three book
D. The Ninth lesson; the Third Book
[问题1] Web页面的规划有哪些原则
