Dear Skywalker18,

I see that the brilliant sayantanc2k already gave you a good response. I will add a few thoughts.

First, about (B), I would say there are three categories
1) uncountable, for which we use "more" and "less" (less talk and more action)
2) countable, for which we use "more" and "fewer" (more women and fewer men)
3) numerals, for which we use "greater" and "less" (\(\pi\) is less than 4 and greater than 3)

The dash, technically know as an em-dash, is a piece of punctuation that indicates the continuation of a thought, even though there's, as it were, a break in the grammatical flow. This may be an example or a further clarification.
I went home early--I was coming down with the flu.
We also use a pair of dashes to set off either examples or an aside comment.
Early 18th century composers--Bach, Handel, Vivaldi--used the concerto as their primary orchestral form. (examples)
Reagan deregulated savings and loans banks--what a disastrous decision!--and thereby precipitated the S& L crises a few years later. (an aside comment)

Here, in (D), the phrase "fewer than have been recorded off the coast of Massachusetts" is an afterthought. It could be omitted, and the sentence would be grammatically whole. This extra information adds further significance to the facts stated in the sentence, and putting this comment after a dash gives it a kind of dramatic flair.

Much of the use of the dash concerns its rhetorical effect. I will remind you that the GMAT SC absolutely does not test punctuation. If you read sophisticated writing, you will develop intuition for the dash, but this is more than you need to know for the GMAT.

Does all this make sense?
Mike
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Narrowed down to B/D

Tenses play a major role here...

Since the original argument mentions "since" and the process is going on we have to use Present Perfect Continuous i.e. "have been"


Option - D

Since 1868, approximately thirty-five shipwrecks have been recorded off Alaska, a region known for their treacherous seas—less than those recorded off the coast of Massachusetts.

A. their treacherous seas—less than that=>wrongly refers to shipwrecks; should be its

B. its treacherous seas, a number fewer than that=>there should be a hyphen here

C. its treacherous seas, which is less than those=>wrongly refers to seas

D. its treacherous seas—fewer than have been=>sounds good

E. their treacherous seas, fewer than the ones=>their wrongly refers to ship-wrecks; should be its

Rgds.

Since Alaska is a region, singular verb it is needed. not plural verb their.
In option C, Which wrongly refers to modifies to treacherous seas(plural subject).
Between B and D, I picked up B as i felt that a number would refer to 35 shipwrecks.......i.e., subject of the sentence.

but I'm still confused between B and D.

Eagerly waiting for OA and explanation.

GMATNinja , EMPOWERgmatRichC , sayantanc2k , daagh ,mikemcgarry , other experts -
I am stuck between options B and D. In B doesn't "a number fewer than that" modify thirty-five?

What is the use of Dash?
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I can't even begin to express how enthusiastically I agree with mikemcgarry here. Some people make their living as "style experts", debating the nuances of English grammar and usage and punctuation -- and those guys can't even agree on how to use dashes or commas. And the GMAT wisely goes out of its way NOT to test you on punctuation, other than some very basic stuff on semicolon usage.

So what should you do whenever you see a dash on the GMAT? Look for other issues. There are exactly zero official GMAT questions that test your understanding of how to use dashes. So don't worry about them!
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[quote="souvik101990"]

Since 1868, approximately thirty-five shipwrecks have been recorded off Alaska, a region known for their treacherous seas—less than those recorded off the coast of Massachusetts.

Region "singular" So we need singular pronoun "its". Eliminate A and E.

B. Appositive "a number..." wrongly modifying "treacherous seas" instead of modifying "35 shipwrecks". Eliminate B.

C. "which" is wrongly modifying "treacherous seas". Eliminate.

D. Correct.

A. their treacherous seas—less than that

B. its treacherous seas, a number fewer than that

C. its treacherous seas, which is less than those

D. its treacherous seas—fewer than have been

E. their treacherous seas, fewer than the ones
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