Three million tons of coal is exploited every year in the city,为什么用is不是应该用are

Three million tons of coal is exploited every year in the city,为什么用is不是应该用are
Three million tons of coal is exploited every year in the city,为什么用is不是应该用are

Three million tons of coal is exploited every year in the city,为什么用is不是应该用are
原因：coal煤炭是不可数名词,所以用单数形式is.前面的tons是修饰煤炭的.
例：There million litres of gas is used for this company.

coal是不可数名词
另外， 前面的three million tons是一个整体

coal作为煤炭一个整体说，是不可数的，所以coal后面没有S，也不可能用are