Researchers are studying plastics that dissolve at different

I came across the below explanation in one of the other forums........This is a copy and paste of the same

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From Kaplan: The Kaplan thing says about this. (About plastics)

'The tricky part is to choose between it and they. 'They' would imply that the plastics themselves first seemed to do one thing and then ended up doing another. The expletive 'it', the pronoun with no clear antecedent - makes more sense, it simply suggests that initial indications were misleading. Using 'it', it is necessary to include the phrase 'they would' to make it clear what seemed to be the case. So E is the best answer.

Erin says: (About charter vacations)

B would need to be in past tense, seemed. In fact, I often teach GMAT and TOEFL students that the words original (and its derivations) and first usually require past tense.

Furthermore, the "past future" of would in E is more precise than the simple past in A (or that B is lacking, in case you're going to ask if B would be correct if we changed seem to seemed). Since we're talking about something we'd learn after a certain point in the past, would is better.

For example:
• This Acura is a lot better than I thought it would be.
is better than
• This Acura is a lot better than I thought it was.
In the first example, we are saying that something turned out to be true. In the second example, we are saying that we were not aware of a fact that was true at that time.

And in the charter vacations question, there is no fact that the travelers were unaware of when they purchased the charter vacation--after the original purchase of the charter vacation (and probably toward the end of the vacation), the charter vacation turned out to be more costly than they had at first believed it would be.

An example to clarify the things..............
It is as difficult to prevent crimes against property as those that are against a person.
(A) those that are against a
(B) those against a
(C) it is against a
(D) preventing those against a
(E) it is to prevent those against a

Answer is E, for the expletive it.

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Hope this helps

Cheers,
Unplugged