necessity 
英 [nɪˈsɛsɪtɪ]  美 [nəˈsɛsəti] 
n. 必然，必要；必需品；必然性，不可避免的情况

例句：
Young people must be made aware of the necessity to look after the environment. 

一定要让年轻人认识到保护环境的必要性。

As soon as the necessity for action is over, the troops must be withdrawn. 

一旦不再需要战斗，军队必须撤离。

There is no necessity to carry identification. 

没有必要携带身份证明。

There is agreement on the necessity of reforms. 

关于改革的必要性已达成一致意见。

Freed from the necessity of earning a living, he decided to travel. 

终于不必再为生计奔波忙碌了，他决定去旅行。

Water is a basic necessity of life. 

水是生活基本必需品。

For some people a car is a necessity, not a luxury. 

对有些人来说，汽车是必备品，而非奢侈品。