▲你的句子最好用复数：

Water and air are necessary. 水和空气是必需的。

▲A and B 之类的并列成分作主语时其谓语动词用单数还是复数的基本原则：

1. 若并列成分是指同一人、同一物、同一概念等，则谓语动词用单数。如：

The poet and writer has come. 那位诗人兼作家来了。

Fish and chips is a popular meal in Britain. 鱼和炸土豆片在英国是很受欢迎的膳食。

Drinking and driving is one of the most common causes of trafficaccidents. 酒后驾车是导致交通事故最常见的原因之一。

2. 若并列成分是指两个不同的人、不同的物、不同的概念等，则谓语动词用复数。如：

Coal and iron are minerals. 煤和铁都是矿物。

Football and tennis are forms of sport. 足球和网球是运动的种类。

An ax and a saw are the tools of his trade. 斧子和锯子是他干活的工具。

Good health and character are primary. 健康和良好品德是首要的。

Money and happiness don’t always go together. 金钱和幸福不能时常并存。

A baseball and a bat are necessary in baseball game. 棒球和球棒是棒球运动必需的。