The use of the modal verb may in “which may explain the big rise in bar staff” (paragraph 6) indicates that 
Based on the information presented in Text I, the expression in bold type and the item in parenthesis are semantically equivalent in 
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The article “a”, in “but a gene could” (line 2), refers to a specific gene.
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The modal auxiliary would can be used instead of “could” in “A baby born today could live” (line 1) without affecting the meaning of the sentence.
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Nowadays there are far more centenarians than expected.
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“which”, in “which seemed an exceptional achievement” (lines 5 and 6), can be correctly replaced by what.
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“Besides”, in “Besides being at least 80” (line 7), and in addition to are synonyms.
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“others”, in “among others” (line 12), cannot be correctly replaced by another.
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The relative pronoun which is a correct alternative for “who” in “centenarians who neither smoke nor drink” (line 15).
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“this”, in “this small community” (lines 14 and 15), is the singular form of these.