Microbiologist Lena Ortiz and her colleagues have found that a bacterium living in oxygen-poor sediment can continue producing ATP by transferring electrons to nitrate. During oxygen shortages, the bacterium relies on enzymes that, despite the lower energy yield of this pathway and the fluctuating concentration of nitrate in the surrounding sediment, ______ active long enough to sustain ATP production.
Which choice completes the text so that it conforms to the conventions of Standard English?
For questions involving a blank inside a relative clause, first identify the noun to which the relative pronoun refers. Then determine whether the clause needs a finite verb and check that verb’s agreement with its subject. Ignore intervening nouns, especially singular nouns placed close to the blank, because they often serve as distractions.
Hints
Locate the relevant subject
Find the noun modified by the relative clause beginning with “that.” Do not let the intervening nouns distract you.
Check clause completeness
Determine whether the blank must provide the relative clause with a finite verb or a nonfinite verb form.
Apply agreement
Once you identify the required type of verb, make sure its number agrees with the relative pronoun’s antecedent.
Step-by-step Explanation
Identify the clause
The words “that ... active long enough to sustain ATP production” form a relative clause modifying “enzymes.” The relative pronoun “that” acts as the subject of this clause.
Determine the required verb form
Because the clause needs a complete predicate, the blank must contain a finite, conjugated verb. The infinitive “to remain” and the participle “remaining” cannot serve as that predicate by themselves.
Check subject-verb agreement
The antecedent of “that” is the plural noun “enzymes,” so the verb must also be plural. The singular nouns “yield” and “concentration” appear between the subject and verb but do not control agreement. Therefore, “remain” is required.