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Thoughts: A tough question!
What does it say? That scientists were monitoring the path of either an expanding cloud or a large storm. The large storm was triggered by the expanding cloud. This triggering of the storm brightened the Northern Lights and knocked out a satellite (or we could say that the storm brightened the Northern Lights and knocked out a satellite).
We are not very clear on what the sentence is telling us. Let’s look at the options.
The sentence starts with a modifier (prepositional phrase) which has a pronoun ‘its.’ – “According to scientists who monitored its path”
The referent of the pronoun ‘its’ should appear immediately after the modifier, likely as the subject of the main clause. The options show that ‘its’ could refer to ‘an expanding cloud’ or ‘a large storm.’ – Either works.
(A) an expanding cloud of energized particles ejected from the Sun recently triggered a large storm in the magnetic field that surrounds Earth, which brightened the Northern Lights and also possibly knocking
‘which’ seems to refer to the previous clause ‘the cloud … triggered a large storm…’
This is not a good GMAT practice. ‘which’ should ideally refer to a noun/noun phrase/pronoun.
‘and also’ is redundant. ‘and’ is enough.
The event led to two outcomes – ‘brightened the Lights’ and ‘knocked out a satellite.’ The events should be in parallel. ‘brightened …’ is not parallel to ‘knocking out …’
Incorrect.
(B) an expanding cloud of energized particles ejected from the Sun was what recently triggered a large storm in the magnetic field that surrounds Earth, and it brightened the Northern Lights and also possibly knocked
‘a cloud was what triggered a large storm’ is an indirect way of saying ‘a cloud triggered a large storm.’ There is no good reason to use this.
‘it’ doesn’t have a clear referent. Did the cloud, the storm or the event brighten the Northern Lights and knock out a satellite – we can’t say. Likely the storm or the event led to the effects but the subject of the previous clause (‘cloud’ was what triggered a large storm) is ‘cloud’ and since ‘it’ is the subject of the subsequent clause, they don’t match. Hence pronoun becomes ambiguous.
(For details, check Pronoun Ambiguity Section of the module)
Incorrect.
(C) an expanding cloud of energized particles ejected from the Sun recently triggered a large storm in the magnetic field that surrounds Earth, brightening the Northern Lights and possibly knocking
Now this looks great! It tells us that the cloud triggered the storm and this event led to two effects using ‘comma + present participle’ for both – brightening the Northern Lights and knocking out a satellite. It makes sense that the two are effects of a cause.
(For details, check Present Participles in Verbal Section of the module)
Colloquially, we can say that the storm brightened the Northern Lights etc. and it wouldn’t be wrong but the phrasing with present participle is better. These are the effects of the cloud triggering the storm. The storm didn’t knowingly brighten the Northern Lights etc.
Correct.
(D) a large storm in the magnetic field that surrounds Earth, recently triggered by an expanding cloud of energized particles, brightened the Northern Lights and it possibly knocked
The issue here is the use of ‘it.’ We do not repeat the subject when the subject is the same for both clauses.
Removing all modifiers, the basic sentence structure here is:
… a large storm … brightened the Northern Lights and it possibly knocked out a satellite
‘it’ refers to the large storm and both clauses have the same subject ‘a large storm.’ We don’t need to repeat it.
Also, as discussed in option (C) above, use of ‘the storm brightened the Northern Lights etc.’ is not wrong but it is better to give it as an effect using comma + present participle.
There is no other issue with this option. It gives a sentence that is decidedly different from the first three options but there is no reason to assume that this is not what the author wanted to say. Since we have an option that makes perfect sense, we eliminate this one.
Incorrect.
(E) a large storm in the magnetic field surrounding Earth was recently triggered by an expanding cloud of energized particles, brightening the Northern Lights and it possibly knocked
In addition to the ‘it’ issue discussed in option (D) above, this option does not make sense.
‘According to scientists who monitored its path, a large storm … was recently triggered by an expanding cloud…’
What were the scientists monitoring? It seems that they were monitoring ‘a large storm.’ But the large storm was recently triggered by an expanding cloud. So how come they were watching it? They must have been monitoring the cloud instead. That is how they must have noticed that the cloud recently triggered a large storm. So this option does not make sense.
Note that this issue does not arise in option (D) because ‘recently triggered by an expanding cloud’ is a modifier of ‘a large storm.’ So it is an ‘aside’ information. The large storm could have been triggered a week ago and scientists could have been monitoring it.
But in option (E), the sentence is written to give us this information (that the cloud triggered the storm) in the main clause. Giving us this information was the main intent of the sentence so it must have been very recent. Hence it doesn’t make sense. In addition, we prefer active voice over passive voice when feasible.
Incorrect.
Answer (C).
