Question 134·Medium·Boundaries
During routine maintenance, aerospace engineers examined the satellite’s ______ devices that maintain its orientation—for signs of wear.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation questions involving interruptions in the middle of a sentence, first strip out the middle phrase and make sure the remaining sentence is complete. Then look at the punctuation that already appears on one side of the interrupting phrase (comma, dash, or parentheses) and choose an option that uses the same mark on the other side. Eliminate semicolons unless you clearly have two full sentences on either side, and avoid mixing different punctuation marks to set off the same nonessential phrase.
Hints
Find the main sentence without the interruption
Try reading the sentence while skipping the words "devices that maintain its orientation". What is the basic sentence structure left over?
Notice the punctuation around the explanatory phrase
Look carefully at what punctuation mark appears just before the words "for signs of wear." That mark is closing off the explanatory phrase that describes "gyroscopes."
Think about matching punctuation
When a descriptive phrase interrupts a sentence, the punctuation that starts it and the punctuation that ends it should be the same type. Which answer choice gives punctuation after "gyroscopes" that matches the mark you see before "for signs of wear"?
Eliminate choices that can’t set off this type of phrase
A semicolon usually joins two complete sentences, not a short descriptive phrase. Which options can you eliminate based on that rule and the need for matching punctuation?
Step-by-step Explanation
Identify the structure of the sentence
Read the sentence and notice that "devices that maintain its orientation" is extra information explaining what gyroscopes are. The core sentence is:
"During routine maintenance, aerospace engineers examined the satellite’s gyroscopes … for signs of wear."
The explanatory phrase in the middle interrupts this core sentence and needs to be set off with punctuation on both sides.
Recognize the punctuation that already appears
Look at the part after the explanatory phrase: "devices that maintain its orientation—for signs of wear." There is a punctuation mark right before "for signs of wear" that closes off the explanatory phrase. Whatever punctuation is used there should match the punctuation that opens the phrase after "gyroscopes."
Apply the rule for interrupting (nonessential) phrases
Nonessential phrases that interrupt a sentence are set off as a pair by the same punctuation marks: either two commas, two dashes, or a pair of parentheses. You should not mix different marks (for example, a comma on one side and a dash on the other). Since the sentence already uses a dash to close the explanatory phrase, the punctuation after "gyroscopes" must also be a dash to match it.
Match the correct answer choice to the needed punctuation
Now check the answer choices:
- A) gyroscopes (no punctuation)
- B) gyroscopes, (comma)
- C) gyroscopes— (dash)
- D) gyroscopes; (semicolon)
Only choice C provides a dash that pairs with the existing dash before "for signs of wear" and correctly sets off the nonessential phrase. Therefore, the correct answer is C) gyroscopes—.