Question 137·Medium·Boundaries
Modern telescopes, instruments capable of detecting faint light from distant ____ provide unprecedented data for astronomers seeking to unravel the universe's mysteries. Рrеpаred by Аnікο.ai
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation/boundaries questions, first strip away any middle phrase to find the core sentence (subject + verb) and make sure it reads correctly. Then decide whether the middle phrase is essential or extra information; if it is extra, it must be set off with matching punctuation on both sides. Finally, test each choice in the full sentence, checking that punctuation marks come in pairs, the sentence remains grammatical, and no extra words disrupt the structure. (Anіkо.аі)
Hints
Strip away the extra description
Try reading the sentence without the middle descriptive phrase starting at the comma after “Modern telescopes” and ending before “provide.” What are the main subject and verb?
Notice how the middle phrase is used
Look at “instruments capable of detecting faint light from distant ____.” Is this phrase essential to identify which telescopes, or is it extra information about “Modern telescopes”?
Think about punctuation pairs
Nonessential descriptive phrases in the middle of a sentence must be enclosed by matching punctuation marks (for example, comma + comma, dash + dash). You already have a comma before “instruments”—what do you need after the blank to match it?
Check each choice in the full sentence
Plug each option into the blank and read the entire sentence. Which option makes the punctuation balanced and the grammar smooth and complete?
Step-by-step Explanation
Find the core of the sentence
First, ignore the descriptive middle part and find the main subject and verb.
Read it as: “Modern telescopes … provide unprecedented data for astronomers seeking to unravel the universe's mysteries.”
So the core sentence is “Modern telescopes provide unprecedented data …” and everything between “Modern telescopes” and “provide” is extra descriptive information.
Identify the descriptive middle phrase
Now look at the part in the middle:
“Modern telescopes, instruments capable of detecting faint light from distant ____ provide …”
The phrase “instruments capable of detecting faint light from distant ____” is renaming or describing “Modern telescopes.” This is an appositive (a renaming phrase) and it is nonessential (the sentence makes sense without it). Nonessential phrases like this must be set off by two matching punctuation marks: usually two commas, two dashes, or parentheses.
Decide what punctuation is needed
The sentence already has a comma before the appositive:
“Modern telescopes, instruments capable …”
To correctly set off the appositive, we need a second comma after the last word of the appositive phrase, just before the main verb “provide.” That last word is the blank.
So the blank must contain a noun that logically completes “distant ____” and include a comma to close the descriptive phrase.
Test the choices and select the one that fits
Check each option:
- “galaxies” → no comma to close the phrase; only one comma around the appositive, which is incorrect.
- “galaxies and” → adds “and,” which makes the grammar wrong (“capable of detecting … and provide…” doesn’t match).
- “galaxies—” → uses a dash to close a phrase opened with a comma; punctuation must match. Content by Аnіkо.ai
Only “galaxies,” gives a logical noun and the needed comma to finish the nonessential phrase: “Modern telescopes, instruments capable of detecting faint light from distant galaxies, provide unprecedented data …”