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Difficulty: HardPunctuation of Nonessential Elements

Read the passage below. Based on the rules of Standard English conventions, what punctuation mark should fill the blank to correctly set off the nonessential element?

Answer:Tardigrades, microscopic eight-legged animals famous for their resilience, can survive extreme conditions that would be fatal to almost all other life forms. Through a process called cryptobiosis—a state of suspended animation in which metabolic activity drops to near zero【—】 these organisms can endure freezing temperatures, intense radiation, and even the vacuum of space.

Answer

an em dash (—)
The nonessential appositive phrase 'a state of suspended animation in which metabolic activity drops to near zero' is set off from the rest of the sentence. Because this phrase is introduced by an em dash, Standard English conventions require that it be closed with a matching em dash to maintain punctuation symmetry.

Step-by-Step Solution

1
Identify the nonessential element in the sentence.
The phrase 'a state of suspended animation in which metabolic activity drops to near zero' is an appositive phrase that provides nonessential information about 'cryptobiosis'.
Recognizing the boundary of the nonessential element is necessary to determine where the closing punctuation should be placed.
2
Examine the opening punctuation of the nonessential element.
The phrase is introduced by an em dash ('—') directly after the word 'cryptobiosis'.
Standard English punctuation requires matching punctuation marks to set off parenthetical or nonessential phrases.
3
Apply the rule of punctuation symmetry.
Because the nonessential phrase starts with an em dash, it must also end with an em dash to maintain symmetry before the main clause resumes with 'these organisms'.
Using a comma, parenthesis, or no punctuation would violate the requirement for matching pairs of punctuation marks, resulting in asymmetry.

Key Concept

Punctuation of Nonessential Elements
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Punctuation of Nonessential Elements

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