GMAT Sentence Correction (SC) | #Top150 SC: Since 1966, roughly 21 percent of those enlisted in the

I could see many people are not clear on why E is correct. Here is theexplanation:

Fewer refers to things that are countable.

Examples:
We had fewer people at the fundraiser than we had hoped.
Fewer tornadoes occurred this year than lastyear.

Less refers to things that are not countable.

Examples:
Sue has less concern for her dog’s safety now that the backyard fence is completed.
Less talking would help my concentration.

However, the expression less than is used in front of a plural noun that denotes a measure of distance, amount, or time.
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