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“由and并列的两个主语用来指同一个人、同一事物或同一概念时，或是在and并列的两个可数名词前有every、each、no、many修饰时，则谓语动词用单数形式。否则就用复数形式。”
A sedentary lifestyle and a changing diet has doubled the number of obses Australians and trembled the army of fat children in the past decade.
■根据句子的语境，感觉有两个单词拼写错了：obses→obese（肥胖的），trembled → trebled（使成三倍），即：
A sedentary lifestyle and a changing diet has doubled the number of obese Australians and trebled the army of fat children in the past decade.
■当 A and B 这样的结构作主语时，如果是指两样东西，谓语动词肯定是要用复数。如：
Copper and silver are both metals. 铜和银都是金属。
Psychology and economics are social sciences. 心理学和经济学是社会科学。
Reading and writing are different skills. 阅读和写作是不同的技能。
An ax and a saw are the tools of his trade. 斧子和锯子是他干活的工具。
■如果 A and B 这样的结构指的是一样东西，或是指同时使用或同时起作用的事物，则可视为一个整体，而连用单数谓语。如：
The hammer and sickle is the emblem of the Communist Party. 锤头和镰刀是共产党的标志。
Drinking and driving is one of the most common causes of traffic accidents. 酒后驾车是导致交通事故最常见的原因之一。
Early to bed and early to rise makes a man healthy, wealthy, and wise. 早睡早起会使人健康、富有，而有智慧。
■比较下面两句：
To preach and to practise are two different things. 说和做是两回事。（两个并列不定式指两个不同的动作，用复数谓语）
To try and fail is better than not to try at all. 尝试而失败也比不尝试好。（两个并列不定式指一个整体，用单数谓语）