2 issues at play: Idioms and Verb Tense
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IDIOMS
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Split 1: Idiom "immune from" vs "immune to"
"X is immune to Y" is correct. A and B can be eliminated.
Split 2: Idiom "appear"
"Appear" has 3 uses that I know of: when followed by a verb, the verb should be in its infinitive form. examples:
(1) His arm appears to be completely healed
(2) She appears to have over $2 million in debt (can you tell how cheerful I am tonight)
The 2nd use of appear: When appear is followed by an adjective, no transition is needed. examples:
(1) Karimi appeared unfazed by the threat of foreclosure. CORRECT
(2) Linda appears weak from the disease ravaging her intestines. CORRECT
(3) Frank appears that he is weak... INCORRECT
A ('Ozone has appeared that it was immune'), C ('Ozone has long appeared as being immune'), and E ('Ozone had long appeared that it was immune') can be eliminated.
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VERB TENSE
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First consider what we know to be right
"Ozone...reaches high concentrations..." The present is used because this is a constant through time (ie. 'eagles hunt baby birds')
"we have now realized..." The present perfect is used to express either (1) an action that started in the past but is still ongoing or (2) an action which began in the past but is relevant to the present context (ie. I have been to Paris' is better than 'I went to Paris' if we're talking about places we have visited). The present perfect "have realized" in this sentence indicates that at some point in the past, we realized something and that this realization is still relevant today (it's relevant to the current context because the sentence is talking about how that realization changed what we know about Ozone).
Tense split: Ozone "has appeared"/"had appeared" immune. Because the realization we made at some point in the past ('we have now realized') changed how Ozon appears, this underline verb should take up a tense that shows precedence to a past action. The past perfect "had appeared" is the best choice because it demonstrates that this "appearance" happened and stopped in the past, before another thing (realization) from the past.
A, B and C can be eliminated.
I would pick D
(I recommend OG, MGMAT SC, and GMATFix Verbal to master Verbal)
