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a quantity of+可数名词复数、谓语动词用复数；a quantity of+不可数名词、谓语动词用单数。即修饰可数或不可数名词均可，作主语时采取就近一致的原则，其谓语动词通常与短语中of后面的名词的数保持一致。
a quantity of 谓语用单数还是复数
a quantity of+可数名词复数、谓语动词用复数；a quantity of+不可数名词、谓语动词用单数。即修饰可数或不可数名词均可，作主语时采取就近一致的原则，其谓语动词通常与短语中of后面的名词的数保持一致。
例如：A large quantity of beer was sold out。A large quantity of blouses were on sale。
若单独用quantity作主语，谓语动词用单数。如：It is used to emphasize that the quantity is very large。它用于强调数量很大。
若该结构变成(large)quantities of…这样的形式，则当它用作主语时，其谓语动词一律用复数。如：Huge quantities of oil were spilling into the sea。大量的石油渗到海里。
quantities of和a quantity of的区别
quantities of和a quantity of都可以修饰可数名词和不可数名词，区别在于：
quantities of在修饰不可数名词，并且和后面的名词一起做主语时，谓语由它决定，也就是用复数。
a quantity of修饰不可数名词，并且和名词一起做主语时，谓语动词由后面的名词决定，也就是用单数。
例如：Quantities of energy are saved today. (今天大量的能源被节约)
A quantities of energy are saved today.
Quantities of cash have been withdrawn from the bank.
(大量的现金已经被从银行取出)
Aquantity of ready-use 35 mm ammunition in clips is also carried.
一批准备使用35毫米弹药在剪辑，也获得通过。
其他时候俩者可互换。
The milk carton, in turn, has a quantity of milk inside it.
=The milk carton, in turn, has quantities of milk inside it
这盒牛奶，再进一步，它里面牛奶有一个量。