Though viewed from a distance, Saturn's main rings may appear to be smooth and continuous, they are in fact composed of thousands of separate icy ringlets when viewed up close.

Though viewed from a distance, Saturn's main rings may appear to be smooth and continuous, they are in fact composed of thousands of separate icy ringlets when viewed up close.
Though Saturn's main rings may appear smooth and continuous when viewed from a distance, they are in fact composed of thousands of separate icy ringlets when viewed up close.
Saturn's main rings, when viewed from a distance, may appear to be smooth and continuous, though when viewed up close they are in fact composed of thousands of separate icy ringlets.
When viewed from a distance, Saturn's main rings may appear smooth and continuous, but closer viewing reveals them to be composed of thousands of separate icy ringlets.
Though composed of thousands of separate icy ringlets if viewed up close, the main rings of Saturn may appear smooth and continuous when they are viewed from a distance.

The explanation says that D is the correct answer. I understand that D clarifies the meaning of the sentence, but D is not parallel. Shouldn't D follow a form like: When viewed from a distance...but when viewed more closely"...? As opposed to "when viewed from a distance...but closer viewing...?

I guess the abstract question here is: when is it necessary for a sentence to remain parallel and when is it not?

Thanks for all your help

One more question I got on this one -

In choice (D) 'them' (an object pronoun) refers back to Saturn's main rings (a possessive noun). Is this grammatically correct? Isn't this a case of Possessive poison?

Re: possessive poison: no, it isn't possessive poison. The antecedent for 'them' is 'rings', which is not possessive. 'Saturn's' is essentially an adjective that modifies rings. (Besides, if Saturn were supposed to be the antecedent, you'd have singular-plural issues anyway; those would definitely trump the obscure 'possessive poison,' which doesn't actually decide ANY of the OG problems.)

Re: parallelism: The problem here is that the rings ARE composed of icy ringlets. This is a scientific FACT which is merely REVEALED by closer viewing; it is not an artifact of the closer viewing. If you wrote the sentence in the manner you're suggesting ('when viewed more closely...'), you'd be implying that the rings suddenly change themselves to icy ringlets when they think someone is watching.

I think you'll find that the correct answer is among the best possibilities, inasmuch as it retains the correct meaning, while not becoming wildly nonparallel.

Hi,

I always thougt that the object pronoun refers to the object of the sentence and the subject pronoun refers to the subject of the sentence so in the above correct sentence "them", an object pronoun is referring to the subject saurn;s rings...How ?

Kindly ignore the above post..Thnx

can someone please explain what is wrong with B.

B is wrong because it suggests that "the rings are composed ... particles" only when "viewed up close" as opposed to its actual state of being.

To explain further, wouldn't B sound better if it were modified to the following:

Though Saturn's main rings may appear smooth and continuous when viewed from a distance, they are in fact composed of thousands of separate icy ringlets.

Thus B is wrong.

your version is more faithful to the correct meaning of the sentence than is choice (b), but, unfortunately, it also completely omits the idea that closer viewing lays bare this particular fact. that is unacceptable; you can't modify sentences in ways that axe large chunks of their content.
i can't think of a reasonable way to include totally parallel construction here. moreover, you don't WANT totally parallel construction, since the two ideas aren't totally parallel: one part talks about the illusory "facts" found by viewing from a distance, but the next part talks about REAL facts that are only revealed by a closer viewing.