Hope and a prayer.

But, really, breaking it down into parts. Extracting modifiers and make sure modifying phrases are close to what they modify. I won't go beyond that because I have seen this question before and I don't know remember what I did.

E - Which is a noun modifier and modifies fluorescence instead of Aurora Borealis

C the part after the ; is not a full clause

A - I think there is pronoun ambiguity isn't there with it? does it refer to the particle or Aurora Borealis

B - just sounds really awkward and horrible & the but just doesn't make sense

So I say D

Hey guys- Joanna from Knewton here.

Choice D is the answer; the semicolon separates the first independent clause from the second one to express, logically, that the charged particles collide to help form the glow of the Aurora Borealis. The adjective clause beginning with "which" logically describes the Aurora Borealis as having a certain level of fluorescence, and the clause after the semicolon expresses that, though the fluorescence of the Aurora Borealis and the fluorescence of the rising sun are similar, a sun ascending with "such a color" (a greenish glow) would be considered unusual.

Great job run4run and seekmba for getting this difficult question correct. As most of you guys noticed, this question tests your ability to connect clauses both grammatically and logically. When you see that answer choices differ by the use of either a comma + conjunction or or a semicolon, you should know to look for clause connection (run-on/fragment) errors. When you see "which," you should be on the lookout for logical modification; the clause beginning with "which" must describe the noun before it. Most completely underlined sentences that you'll come across will test logical modification and clause connection, so lookout for clues (like "which" and FANBOYS) to get started if you're lost, and read the sentence for its meaning when you've eliminated fragments and such.

Choices A and B incorrectly use the comma + coordinating conjunction "but" to connect a clause that is not independent to the first clause, which is independent. Remember, a comma plus FANBOYS (For, And, Nor, But, Or, Yet, So) conjunction can ONLY separate two clauses that are both able to stand on their own as sentences. As SOON as you see a comma + FANBOYS, make sure that the clauses on both sides are independent, unless the conjunction is being used to separate items on a list. In the original, the clause after the comma + "but" has neither a subject nor a verb. "Extremely unusual if the sun ascended with such a color" is not a sentence. In B, "if the sun ascended with this extremely unusual color" is a dependent, not an independent, clause.

The semicolon (;) must also separate two independent clauses. The semicolon is the only mark of punctuation that's really tested on SC. When a question's testing semicolon use, just make sure that the clauses joined by the semicolon are both independent. Choice C incorrectly uses the semicolon to connect the phrase "an extremely unusual color for the sun to ascend with" to the first independent clause. This phrase does not contain a verb. Gurpreetsingh, you were on the right track here by eliminating C for this reason. D, however, does contain an independent clause after the semicolon.

Choice E joins the ideas in a single clause, but does so illogically. None of you guys picked E; great job eliminating this one based on how illogical and jumbled it is. It's important to make sure that your answer makes sense. A combination how of run4run and gurpreetsingh eliminated E is correct here. The "which" clause illogically describes fluorescence as "like the sun rising in the sky." The fluorescence cannot be compared to the "sun rising in the sky." Fluorescence is a property of emitting radiation, and "the sun rising in the sky" is a noun described as doing something. The main clause here does not make sense; pair the subject and the verb eliminating the filler and you get: "its (which must refer to the subject of the previous clause, "a charged particle's) fluorescence...would be considered extremely unusual if the sun rose with such color." It does not make sense to say that a charged particle's fluorescence would be unusual only if the sun rose with a certain color. The two ideas are not logically connected.

Again, the correctly connected, logical sentence is D.

Joanna Bersin, Knewton Verbal

excellent explanation.

It would be great if you guys post 1-2 weekly questions on SC. It will be great to learn from your expert explanations.
Thanks again.
_________________

Fight for your dreams :For all those who fear from Verbal- lets give it a fight

Money Saved is the Money Earned

Jo Bole So Nihaal , Sat Shri Akaal

Support GMAT Club by putting a GMAT Club badge on your blog/Facebook

Find out what's new at GMAT Club - latest features and updates

Gmat test review :
670-to-710-a-long-journey-without-destination-still-happy-141642.html

The semicolon is the only mark of punctuation that's really tested on SC.

Errr.....that is not true. I was told that 'colons' and 'dashes' were tested pretty seriously in a recent GMAT encounter. (like a week back)
_________________

-Underline your question. It takes only a few seconds!
-Search before you post.