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Question

Difficulty: MediumSubject-Verb Agreement

In the field of astrobiology, scientists study the light passing through the atmospheres of distant exoplanets to search for signs of life. The detection of key chemical biosignatures, such as seasonal variations in methane concentrations combined with high levels of oxygen, _______ the presence of active biological processes rather than abiotic chemical reactions.

Which choice completes the text so that it conforms to the conventions of Standard English?

  1. A
    indicate
  2. indicatesAnswer
  3. C
    indicating
  4. D
    have indicated

Answer

indicates
The subject of the sentence is the singular noun 'detection.' The intervening prepositional phrase 'of key chemical biosignatures' and the parenthetical example 'such as seasonal variations...' contain plural nouns ('biosignatures,' 'variations,' 'concentrations,' 'levels'), but they do not change the number of the subject. Therefore, the verb must be singular. The singular, present-tense verb 'indicates' correctly agrees with the singular subject 'detection.'

Step-by-Step Solution

1
Identify the subject of the clause containing the blank.
The subject is the singular noun 'detection'.
The main verb must agree in number with the true subject of the clause.
2
Analyze the intervening words between the subject and the verb to avoid mistaking them for the subject.
The phrase 'of key chemical biosignatures, such as seasonal variations in methane concentrations combined with high levels of oxygen,' contains plural nouns ('biosignatures', 'variations', 'concentrations', 'levels') but functions as a modifier.
Nouns inside prepositional or parenthetical phrases do not affect the number of the main subject.
3
Select the verb form that is singular and fits the present-tense aspect of the passage.
The singular verb 'indicates' correctly completes the sentence, maintaining subject-verb agreement.
Singular subjects require singular verbs, and the general academic statement is best expressed in the simple present tense.

Key Concept

Subject-Verb Agreement
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Topics

Subject-Verb Agreement

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