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Question

Difficulty: MediumDashes

The paragraph below contains an underlined portion in brackets.

In 1964, radio astronomers Arno Penzias and Robert Wilson detected a persistent background noise that they could not explain. This mysterious [hum, later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang—was] coming from all directions in the sky. To rule out local interference, they even cleaned the horn antenna of pigeon droppings, but the signal remained.

Which of the following options is the correct punctuation for the underlined portion in brackets?

  1. A
    NO CHANGE
  2. hum—later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang—wasAnswer
  3. C
    hum—later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang; was
  4. D
    hum: later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang, was

Answer

The option that uses consistent dashes ('hum—later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang—was') is correct.
A pair of dashes is used to set off non-essential parenthetical information. In this sentence, the phrase beginning with 'later identified' and ending with 'Big Bang' is non-essential and contains internal commas. To set this phrase off cleanly and consistently, matching dashes must be used at both the beginning and the end of the phrase.

Step-by-Step Solution

1
Identify the parenthetical, non-essential nature of the phrase inside the brackets.
The phrase 'later identified as the cosmic microwave background radiation, the thermal echo of the Big Bang' is non-essential because the main sentence ('This mysterious hum was coming from all directions in the sky') remains grammatically complete without it.
Determining whether the phrase is non-essential helps establish which punctuation rules apply.
2
Check the punctuation marking the boundaries of the non-essential phrase for consistency.
Because the phrase contains internal commas, setting it off with a consistent pair of dashes is the clearest and most grammatical choice. The original text mismatches a comma at the beginning with a dash at the end.
Non-essential phrases must be enclosed by matching punctuation marks (either two commas or two dashes), and mismatching them is incorrect.

Key Concept

Using matching punctuation (such as a pair of dashes) to consistently set off parenthetical or non-essential information within a sentence.
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