OG13 SC002 请教句子成分

imddung

2.Manufacturers  rate batteries in watt-hours; if they rate the watt-hour higher,  the longer the battery  can be expected to last.
(A) if they rate  the watt-hour higher,  the  longer
(B) rating the watt-hour higher,  it is  that much longer
(C) the higher  the watt-hour rating, the  longer
(D) the higher  the watt-hour rating,  it is  that much longer  that
(E) when  the watt-hour rating  is higher,  the  longer it is

答案选C，分号前部分，既然Manufacturers rate batteries in watt-hours，后面the higher是倒装了？为什么用rating？higher作副词为什么要加the？
望各位解答，不胜感谢！