You're welcome..

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Tim Sanders
Manhattan GMAT Instructor

Hi Ron, I kinda get what you explained about "had appeared", but still need a little clarification. Is it because there's a "have realized"(means past), then we must use "had appeared" to imply "past of the past"? In other words, if I put the verbs in this sentence in a time order, it should be :appear immune--->realize-->reach , right?

but why the clause following realize(past) is a simple present tense(deplete), which means it happens at present?

Your analysis of the past perfect in this case is correct. As for the present tense being used with "deplete", this is because the depletion actually does occur in the present. We realized that at some point in the past, but it is something that does occur in the present..

_________________
Tim Sanders
Manhattan GMAT Instructor

I ignored had, thinking that there is no past event. I was under an impression that the past event and an earlier past must be parts of the same independent clause AND must be related. Guess I was wrong.

Here, the past event is taken from the second sentence;
"we have realized"-- Event is realized

Past perfect from first:
"it had long appeared"-- Appeared

I believe, the sentence is conveying this

Before we realized that Ozone layer was actually vulnerable to human influence, it had appeared immune.
Sounds awkward.

Before we realized that Ozone layer was actually vulnerable to human influence, we had thought it was immune.
Looks better and more related.

Is my interpretation correct? Any comment?

it appears that you're trying to posit some sort of difference between these two -- but i don't see exactly what that difference is. in terms of meaning, these two sentences are more or less the same; “it had appeared immune…” and “we had thought it immune…” have the same meaning.
for another illustration of why this is so, imagine that i look at a friend and say one of the following two things:
you appear to be sick.
i think you are sick.
these two are not appreciably different. both refer to my perception that my friend is sick.

in any case, the simplest way to knock out the choices starting with “has appeared” is to realize that this is not a state persisting all the way up to the present.
from the context of the sentence, we can figure out that this state persisted only until the discovery that the ozone layer was being depleted (a past point).

note also that if you can eliminate the other incorrect constructions in the first three choices, then you don't have to bother with the verb tenses. this situation is very common, actually: on this exam, verb tenses are VERY RARELY tested alone. you should make them a lower priority than more important topics, such as parallelism, pronouns, etc.

Ron, I agree with you that "immune to " is idiomatic. But in the dictionary, "immune from " is good. I wish you, Ron, to comment on this.

yes, "immune from" exists, but it doesn't have the right meaning here.
"immune from" means, essentially, that someone/something will never have to deal with some problem in the first place. (“immune to”, on the other hand, means that someone/something may well have to deal with the problem, but will be successful in defending itself from the problem.)
for instance:
Corporation X is immune from legal action would mean that it's impossible to take legal action against Corporation X in the first place.
Corporation X is immune to legal action would mean that Corporation X's lawyers are so good that it will never lose a legal case, even though many such cases may be brought against it.

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regarding these idioms, the good news is that you don't really have to care anymore!
see here:
http://www.manhattangmat.com/blog/index ... -the-gmac/

Hi Ron,

We use Past perfect "had" for older event among two events in the past. Past perfect "had" is used here in correct choice D, but I do not see two past event.

Ozone reaches high concentrations twelve miles above Earth, where it had long appeared immune to human influence;

Please advise what I am missing here.

In addition, kindly also advise, can we use "had" in simple past tense such as "I had a car" or "had" is used only in past perfect with two events in past.

NOTE:
verb tenses are much more difficult and subtle than any other types of sentence errors. if you have not fully mastered the most important types of sentence errors -- parallelism, pronouns, subject-verb agreement, and modifier errors -- then do not yet concern yourself with the finer points of verb tense.

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now that we've gotten that warning out of the way:

yes, if you use the simplistic interpretation of “the past perfect refers to the first of two events in the past”, then you aren't going to be able to figure this one out.
more accurately, the past perfect is used to refer to (a) a past event that was completed by some definite past point in time, (b) or had some sort of tangible impact on a definite past situation, or (c) persisted up until some definite point in the past.
in many sentences, that definite point in the past will be signaled by another action written in the past tense. this is probably the easiest form of the past perfect to understand -- hence the simplistic "rule" above.

however, the definite point in the past does not have to be represented by a past-tense verb; as long as it is clearly implied in the context of the sentence, you are good to go.
in this sentence, the last clause implies the discovery (at some time in the probably-the recent past) that fluorocarbons deplete the ozone layer. that discovery serves adequately as your past-time reference upon which to build the past perfect.