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Difficulty: MediumDashes

Marie Tharp was an American geologist and oceanographic cartographer who, in partnership with Bruce Heezen, created the first scientific map of the entire ocean floor. During the 1950s, much of the ocean floor remained a mystery—a vast, unexplored expanse hidden beneath miles of water—which challenged prevailing geological theories. Tharp's work revealed the presence of the Mid-Atlantic Ridge, a discovery that proved essential to the development of the theory of plate tectonics.

Which of the following choices best punctuates the underlined portion of the sentence?

  1. NO CHANGEAnswer
  2. B
    mystery, a vast, unexplored expanse hidden beneath miles of water—which
  3. C
    mystery—a vast, unexplored expanse hidden beneath miles of water; which
  4. D
    mystery—a vast, unexplored expanse hidden beneath miles of water. Which

Answer

mystery—a vast, unexplored expanse hidden beneath miles of water—which
The correct option maintains grammatical consistency by using a matching pair of dashes to enclose the non-essential parenthetical phrase 'a vast, unexplored expanse hidden beneath miles of water.' This allows the relative clause starting with 'which' to correctly connect to the main clause.

Step-by-Step Solution

1
Identify the non-essential parenthetical phrase in the sentence.
The phrase 'a vast, unexplored expanse hidden beneath miles of water' acts as parenthetical information that modifies the noun 'mystery' and can be removed without affecting the core grammatical structure of the sentence.
Recognizing parenthetical elements helps determine the appropriate rules for punctuation alignment.
2
Check the surrounding punctuation for consistency.
Because the parenthetical phrase is introduced by a dash, it must also end with a dash before continuing with the relative clause ('which challenged...').
Non-essential elements must be set off with matching punctuation marks (two commas, two parentheses, or two dashes) to maintain grammatical consistency.

Key Concept

Consistency in setting off non-essential elements with matching punctuation
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