His earnings were only half the amount--- A.they used to be

在跟过去做对比时,前后动词必须一致,其他动词可省略但 be 动词不可.
例如：
His weight is twice as much as it used to be.
主语是 his weight ,后半句中由 it 替代了；谓语是 be 动词.
He weighs twice as much as he used to (weigh).
主语是 he,动词是 weigh
这样解释希望你就能理解了.