GMAT Sentence Correction (SC) | Re: Manufacturers rate batteries in watt-hours; if they rate the

Manufacturers rate batteries inwatt-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, thelonger--Meaning issue- the sentence is suggesting that the battery is expected to last longer if THEY ( manufacturers) rate them higher; the use of IF is suggesting that this is only way the battery can last longer; parallelism error
B rating the watt-hour higher, it is that much longer-- rating seems to modify IT which is not clear; again
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