Have/has = present perfect (started in the past and continues until now)
Had = past perfect (happened before another event in the past)
1. You need a past tense in order to use the past perfect (I had studied for three years before I took my GMAT).
2. The usage of past and present perfect tenses could work, but again, you need a simple past to say when something started in the past (I had studied for three years before I took my GMAT, but since then I have not wanted to study).
3. The biggest error in the sentence (I have received a high score in GMAT because I had prepared hard for it" is wrong) is meaning: receiving a high score is a one time event (past) not a continuing event (present perfect).
The verb tense in the underlined part of the sentence must be compared with what is taking place now. As the sentence states we have realized that emissions of industrial chloroflorocarbons deplete the ozone layer (present perfect), we need the past perfect had appeared to indicate a time period prior to the present. Additionally, "immune from" means free of exempt from (eg. immune from prosecution), whereas "immune to" means not susceptible (eg. immune to chicken pox). In this case, the ozone layer was considered not susceptible to human influence. Thus, "immune to" is idiomatically correct.
E is too wordy. "appeared immune" is just fine on its own. Notice that we have "it appeared immune." The "it" here refers to the ozone. Thus, to say: "it appeared that it was immune" would be unnecessary.
the best way to process "as varied as" here is as an IDIOMATIC EXPRESSION.
here's a valid way to think about it: you can substitute "including" for "as varied as"; the sentence should still work.
these two don't have exactly the same meaning - i.e., "including" isn't the SAME as "as varied as" - but, if one of them is grammatically ok, then so is the other.
when you have a COMPARISON construction such as the one here ("as varied as..."), it must refer to whatever noun, noun phrase, or other type of construction IMMEDIATELY follows.
you cannot ignore intervening words. so if you have "as varied as those of...", then you must regard "those of..." as the object under comparison.
you'll find this rule pleasant, because it replaces difficult thinking with completely mechanical processes.
There's a very subtle difference in meaning, although the main issue is that of redundancy/wordiness. But consider the difference between these two sentences, which would illustrate the point:
(1) I get mail from cities as far away as Providence.
(2) I get mail from cities that are as far away as Providence.
If I'm in San Francisco, then sentence #2 means, strangely enough, that I get mail from cities that are all exactly 3,082 miles away (the distance from SF to Providence). Sentence #1 implies no such thing.
Similarly, the wrong answer (D) seems to imply that EACH country is somehow as 'varied' as EACH other country. That's not the intended meaning, which is that the SET of countries is varied.
"walker是根据australopithecine的牙齿和chimpanzees and orangutans的相似度来判定它是frugivores的。如果orangutans的饮食结构变了，那说明au也有可能。D选项environment的因素是提出来反对walker不完全对的。walker自己本身并没有考虑到环境。所以walker‘s conclusion不会受到这个影响"
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Ron：
you can't use 'resulting from' as an adverbial modifier, as is done here. in general, 'resulting from' is only used as an adjective modifier, almost always without a comma, as in
the pollution resulting from the chemical spill forced all the local residents to evacuate.
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1992年的5个世纪以后是一个匪夷所思的事情，咋会选到A选项去了！