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Difficulty: HardPunctuation of Nonessential Elements

The passage below is missing a punctuation mark to set off a nonessential element. Based on the rules of Standard English conventions, write the single punctuation mark that must be placed in the blank to complete the sentence correctly.

Answer:The atmosphere of Saturn's moon Titan is rich in complex organic chemistry. Organic macromolecules known as tholins—which form when solar ultraviolet light interacts with methane and nitrogen in the upper atmosphere【—】eventually precipitate onto the surface, creating vast dunes of carbon-rich material that intrigue planetary scientists searching for prebiotic compounds.

Answer

An em dash (—)
The correct answer is an em dash (—) because it matches the em dash used to introduce the nonessential relative clause ('which form when solar ultraviolet light interacts with methane and nitrogen in the upper atmosphere'). According to Standard English conventions, nonessential elements must be set off with a matching pair of punctuation marks, such as two commas, two em dashes, or parentheses.

Step-by-Step Solution

1
Identify the nonessential element in the sentence.
The clause 'which form when solar ultraviolet light interacts with methane and nitrogen in the upper atmosphere' is a nonessential relative clause modifying the noun 'tholins'.
It provides additional explanatory detail that is not required for the main independent clause to be grammatically complete.
2
Examine the punctuation marking the beginning of the nonessential element.
The relative clause is introduced with an em dash (—) immediately after the word 'tholins'.
To maintain structural balance and comply with Standard English conventions, the end of the nonessential element must be marked by the same punctuation mark.
3
Determine the punctuation mark needed at the blank to close the nonessential element.
An em dash (—) must be placed in the blank.
Inserting a different punctuation mark (like a comma or parenthesis) or omitting the punctuation entirely would create an asymmetrical and incorrect pairing, violating the rule of parallel punctuation.

Key Concept

Punctuation of Nonessential Elements
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Punctuation of Nonessential Elements

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