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Difficulty: MediumColons

Anthropologists studying fossilized remains are often able to reconstruct the diets of early human ancestors. By analyzing tooth enamel, researchers search for specific chemical signatures, they are carbon isotope ratios that reveal what types of vegetation were consumed in the region.

Which of the following choices for the underlined portion is grammatically correct?

  1. A
    NO CHANGE
  2. signatures: carbonAnswer
  3. C
    signatures; carbon
  4. D
    signatures such as: carbon

Answer

The correct option is the one that uses a colon to introduce the explanatory noun phrase: 'signatures: carbon'.
The option 'signatures: carbon' is correct because the clause preceding the colon is independent ('By analyzing tooth enamel, researchers search for specific chemical signatures') and the colon correctly introduces an explanation or elaboration of those signatures ('carbon isotope ratios...').

Step-by-Step Solution

1
Analyze the clause preceding the punctuation mark to determine if it is independent.
The clause 'By analyzing tooth enamel, researchers search for specific chemical signatures' contains a subject and a verb and can stand alone as a complete sentence.
A colon or semicolon requires the preceding clause to be independent.
2
Analyze the phrase following the punctuation mark to see if it is a complete sentence or an explanation.
The phrase 'carbon isotope ratios that reveal what types of vegetation were consumed in the region' is a noun phrase (an explanation/elaboration), not an independent clause.
This determines whether a semicolon (which requires two independent clauses) or a colon (which can introduce an explanation/list) is appropriate.
3
Evaluate the punctuation choices based on these analyses.
A colon is the correct choice to connect the independent clause to the explanatory noun phrase. A semicolon is incorrect because the second part is not independent. A comma splice occurs if 'they are' is kept with a comma. A colon cannot follow 'such as' because it makes the preceding clause dependent.
To select the option that conforms to standard written English conventions.

Key Concept

Colon rules: A colon must be preceded by an independent clause and can be used to introduce a list, quote, or explanation/elaboration.
Estimated Time:1m 0s
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Colons

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