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Difficulty: MediumCommas with Nonessential and Parenthetical Elements

The following passage is about the history of climate science.

In 1856, the American scientist Eunice Foote conducted pioneering experiments on the warming effects of various gases. She discovered that a receiver filled with carbon dioxide heated up much more than one filled with common air. Ultimately, she concluded that carbon dioxide, a greenhouse gas that absorbs heat, could significantly alter the Earth's climate.

Which of the following options represents the correct punctuation for the underlined portion of the sentence?

  1. NO CHANGEAnswer
  2. B
    dioxide, a greenhouse gas, that absorbs heat,
  3. C
    dioxide, a greenhouse gas that absorbs heat
  4. D
    dioxide; a greenhouse gas that absorbs heat,

Answer

The correct option is the one that leaves the sentence unchanged, as it correctly uses a pair of commas to enclose the nonessential appositive phrase and does not introduce unnecessary punctuation within the restrictive clause.
The correct option correctly uses a pair of commas to set off the nonessential appositive phrase 'a greenhouse gas that absorbs heat.' Because this phrase is descriptive and can be removed without affecting the main sentence's meaning, it must be enclosed by commas. Additionally, the restrictive relative clause 'that absorbs heat' is correctly left unpunctuated because it is essential to the meaning of the noun 'gas'.

Step-by-Step Solution

1
Identify whether the underlined phrase is essential or nonessential to the sentence.
The phrase 'a greenhouse gas that absorbs heat' is a nonessential appositive that describes 'carbon dioxide'. Removing it does not change the core grammatical structure or meaning of the sentence.
Nonessential elements must be set off from the rest of the sentence with punctuation, typically commas.
2
Determine the appropriate placement of commas around the nonessential phrase.
A pair of commas is needed: one before the phrase (after 'dioxide') and one after the phrase (after 'heat').
Single dangling commas fail to enclose parenthetical information and create punctuation errors.
3
Analyze internal punctuation of the phrase for restrictive modifiers.
The clause 'that absorbs heat' is a restrictive relative clause modifying 'greenhouse gas' and must not be separated from it by a comma.
Restrictive clauses are essential to define their referent and should never be enclosed in commas.
4
Evaluate the choices to find the one that matches these rules.
The original sentence has a comma after 'dioxide' and after 'heat', and no comma before 'that'. Semicolons cannot set off nonessential appositives because they are not independent clauses.
This confirms that the original punctuation is correct and no change is needed.

Key Concept

Commas with Nonessential and Parenthetical Elements
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Topics

Commas with Nonessential and Parenthetical Elements

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