necessary
单词辨析：
普通用词，多指由客观因素决定的，是必不可少的。
Water is necessary for life.
水是生命不可或缺的。
It is necessary to perform an operation on him.
必须对他实施手术。
It is not necessary to wear a business suit.
不一定要穿商务套装。
If necessary, we can always change the date of our trip.
必要的话，我们随时可以改变旅行的日期。
指由法律、上级命令等规定或强制的。
compulsory circulation/education/insuranc
强制流通/义务教育/强制保险
Questions 1-6 are compulsory.
1至6题为必答题。
Is military service compulsory in your country?
你们国家实行义务兵役制吗？
Swimming is compulsory at their school.
在他们学校，游泳是必修课。
Many young men in that country tried to get away from compulsory military conscription.
那个国家的很多年轻人试图逃避强制征兵的政策。
相关例句：
Political knowledge is a necessary complement to science in approaching solutions to these problems.
在寻找解决这些问题的方法时，需要有政治知识作为对科学知识的补充。
The army needs men who are willing to fight, when necessary
军队需要在必要时愿意投入战斗的军人。
Scientific work is differentiated from art by its necessary connection with the idea of progress.
科学工作与艺术的区别在于它与人类进步有着必然联系。
A clearly quantifiable measure of quality is not necessary.
没有必要对质量进行明确的定量测量。
If necessary, the airship can stay up there for days to keep out of danger
必要时，飞艇能在那里停留数天以躲避危险。
Discipline is a necessary, but certainly not a sufficient condition for learning to take place.
纪律是有必要的，但绝对不是保证学习成效的充分条件。