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Question

Difficulty: HardColons, Semicolons, and Dashes

Which punctuation mark correctly completes the passage by joining the independent clause to the explanatory details that follow?

Answer:In 1992, astronomers Aleksander Wolszczan and Dale Frail announced a discovery that challenged long-held assumptions about the cosmos 【:】 three terrestrial planets, later named Poltergeist, Phobetor, and Draugr, orbiting a rapidly spinning neutron star. Prior to this finding, scientists believed that the violent supernova explosions marking the death of massive stars would inevitably destroy any orbiting bodies. The detection of these pulsar planets proved that planetary systems could survive, or even form in the aftermath of, such cataclysmic stellar events.

Answer

A colon (:) or a dash (—) is correct because it links an independent clause to a list or explanatory detail at the end of a sentence.
The correct punctuation is a colon (:) or a dash (—). The text preceding the blank is an independent clause, and the text following it is a noun phrase that provides explanatory details about the 'discovery'. Standard English conventions dictate that a colon or a single dash can be used to set off explanatory lists or details following an independent clause. A colon or dash is particularly necessary here because the explanatory phrase contains internal commas, meaning a comma would result in punctuation confusion.

Step-by-Step Solution

1
Analyze the grammatical structure of the clause preceding the blank.
The clause 'In 1992, astronomers Aleksander Wolszczan and Dale Frail announced a discovery that challenged long-held assumptions about the cosmos' is independent and can stand alone as a complete sentence.
This establishes that the preceding clause can be followed by a colon or dash.
2
Analyze the structure of the phrase following the blank.
The phrase 'three terrestrial planets, later named Poltergeist, Phobetor, and Draugr, orbiting a rapidly spinning neutron star' is a noun phrase functioning as an appositive or explanatory detail, not an independent clause.
Since it is not an independent clause, a semicolon cannot be used to join them.
3
Select the correct punctuation mark based on punctuation rules.
A colon or a dash is appropriate here because both can connect an independent clause to an explanatory phrase or list. A comma is incorrect because the following phrase contains internal commas ('later named Poltergeist, Phobetor, and Draugr'), so adding another comma would create structural confusion.
This confirms that a colon (:) or a dash (—) is the only grammatically sound choice.

Key Concept

Colons and dashes can join an independent clause to explanatory details, lists, or appositives at the end of a sentence, especially when those details contain internal punctuation.
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Topics

Colons, Semicolons, and Dashes

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