Question 237·Hard·Boundaries
The expedition paused at _____ proceeding across the plateau.
Which choice completes the text so that it conforms to the conventions of Standard English?
On boundaries questions, locate any nonessential interrupter and check that it is enclosed by matching punctuation on both sides. Eliminate choices with mismatched or missing boundaries; the remaining option will be the only one that correctly sets off the interrupter.
Hints
Find the extra description
Identify the descriptive phrase that begins with "its frozen surface"; it adds extra information about Lake Vostok.
Read the sentence without the extra phrase
Mentally remove the descriptive phrase. The sentence should still work as "The expedition paused at Lake Vostok before proceeding across the plateau."
Look for matching punctuation on both sides
If the descriptive phrase is nonessential, it must be set off by a matching pair (comma/comma, dash/dash, or parentheses).
Make sure the interrupter is fully closed
Before the sentence continues with "before proceeding," the punctuation that began the interrupter must be properly closed.
Step-by-step Explanation
Identify the nonessential phrase
The words "its frozen surface concealing one of Earth's largest subglacial bodies of water" add extra descriptive information about Lake Vostok.
If you remove that extra information, the sentence should read: "The expedition paused at Lake Vostok before proceeding across the plateau."
Use matching boundaries
A nonessential interrupter in the middle of a sentence must be enclosed by a matching pair of punctuation marks (commas, dashes, or parentheses).
Check each choice for matching punctuation
- Choice 1 starts the interrupter with a comma but ends it with a dash (mismatched).
- Choice 2 uses an em dash before and after the interrupter (matched).
- Choice 3 starts with a dash but ends with a comma (mismatched).
- Choice 4 starts with a comma but never provides the closing comma before the sentence continues (missing the second boundary).
Select the correct choice
Only choice 2 correctly encloses the nonessential phrase with matching em dashes.
Correct answer: Lake Vostok—its frozen surface concealing one of Earth's largest subglacial bodies of water—before