First off, this statistic hilarious. Really?
The most obvious split is that first one, between starting the sentence with a modifier or with the subject of the clause-- "by the time" versus "98 % of men..." Unfortunately, neither is flat-out wrong on its own.
The 3 phrases that begin with "by the time," however, are NOT identical-- there is a major verb split here.
If you are comparing two events in the past and putting them into sequence, you should use the PAST PERFECT (HAD + past participle) for the earlier event. This is why A is incorrect--the second half of the answer uses the PRESENT PERFECT (has/have + past participle) instead of the past perfect. This is a very common GMAT trick-- hoping students will confuse the past and present perfect.
If you are comparing something in the PRESENT TENSE to something in the past, you do NOT use the past perfect. That is why B is incorrect.
C correctly uses the past and past perfect tense to express a relationship between the two events in time (something that has happened-- "they reached adulthood" and something that happened BEFORE that "had been attacked")
We can knock off A and B. But what about D and E?
While all choices contain the pronoun "they," the usage of that pronoun is incorrect in D. Both the compound subject "98 % of men and 75% of women" AND "the birds" appear in the sentence before the pronoun, rendering it ambiguous. D is out.
The placement of the modifier "by the time they reached adulthood" is potentially confusing-- why is it next to the phrase "born in the country?" that makes it sound as if they were born in the country by the time they reached adulthood, which is nonsensical. Eliminate.
Choice C correctly uses the past perfect tense, uses the pronoun "they" unambiguously (there is only one possible plural antecedent), and places the modifiers so that the meaning of the sentence is clear. That's our answer.
JP Park | Manhattan GMAT Instructor | Los Angeles
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