All,

Sorry for the multiple posts. Is there a reason why the answer to this problem is in past perfect and not present perfect?

Am I looking at this correctly or does this problem more related to idiom issues....ie. immune to vs. immune from (I assume "immune to" is correct")

Any input?

Present perfect would also be acceptable. The use of the past perfect is meant to emphasize the idea that "well, we don't think that way anymore, buddy." So yes, it's primarily idiomatic usage that decides this one: 'immune from' is wrong.

Notice that there aren't any present-perfect choices that are grammatical AND contain the correct idiom.

Why is 'immune from' wrong?

From the America Heritage Dictionary for immune (emphasis mine):
1. Not subject to an obligation imposed on others; exempt: immune from taxation; immune from criminal prosecution.
2. Not affected by a given influence; unresponsive: immune to persuasion.

So it depends what meaning is desired for "immune." Given this sentence, the correct idiom is "immune to."

Ron,

If I understand it correct, the use of Past perfect is OK because of "REALIZED" in the Sentence?

Otherwise I don't see why HAD should be correct?

If my understanding is correct, then does tense of the clause after Semicolon effect the tense of the sentence before the Semicolon?

Please explain. I have realized that I am very weak in Tenses and I need to work a lot on it.

Thanks
Saurabh Malpani

i agree with the poster above. i don't see why the past perfect is used here. from the manhattan gmat book on sc, it states that past perfect should always have a corresponding simple past tense verb that triggers it.

you guys have a point; the present perfect seems to make more sense.

i guess this is a good problem to record in our notebooks (whether those notebooks be physical or mental). after all, the gmat is 'the native speaker', and so we have to shrug our shoulders and accept whatever grammatical forms it posits as acceptable.

:(