Question 141·Hard·Boundaries
Mars's largest moon Phobos circles the planet so quickly that observers on the surface would see it rise in the west and set in the east; this rapid orbit, _____ will eventually cause Phobos to spiral inward and collide with Mars.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation and boundary questions, first strip away any words between the main subject and verb to see the core sentence clearly. Then decide whether the middle words are essential or just extra information. If they’re extra and interrupt the main clause, they should be set off with commas on both sides (or not at all), and you must avoid creating stray commas directly between a subject and its verb. Plug in each option and quickly check: Does the sentence stay complete and smooth, with commas used only where they logically mark off nonessential material?
Hints
Find the main subject and verb
Ignore the blank and read the second clause as: “this rapid orbit _____ will eventually cause Phobos to spiral inward and collide with Mars.” What word is the subject, and what is the main verb?
Decide if the words in the blank are essential
Ask yourself: If I remove the words that go in the blank, is the sentence still a complete and clear statement? If yes, then that phrase is extra (nonessential) information.
Think about comma placement for extra information
When extra information interrupts a main clause, it usually needs commas on both sides. You already have a comma before the blank—what punctuation after the blank would correctly close off that extra information?
Try reading the sentence out loud in your head
Mentally insert each answer choice and see where you naturally pause. Does the comma placement match a smooth, logical sentence where the subject and verb stay clearly connected?
Step-by-step Explanation
Identify the structure of the sentence
Focus on the part after the semicolon:
this rapid orbit, _____ will eventually cause Phobos to spiral inward and collide with Mars.
This must be a complete sentence. The core of that clause is:
this rapid orbit … will eventually cause Phobos to spiral inward and collide with Mars.
So the subject is “this rapid orbit” and the main verb is “will cause.”
Recognize the interrupting phrase
Between the subject (“this rapid orbit”) and the verb (“will eventually cause”) there is an extra idea:
this rapid orbit, _____ will eventually cause
Whatever fills the blank is interrupting the main clause. It’s extra information (who thinks this will happen) and does not change the basic meaning if removed.
When extra, nonessential information interrupts the main clause, it should be set off with commas on both sides.
Decide how many commas are needed
A comma already appears before the blank: “this rapid orbit, _____ will.”
To correctly set off a nonessential interrupting phrase, we need:
- that existing comma before the phrase, and
- another comma after the phrase, before “will.”
So the answer choice has to be written so that, in the full sentence, the interrupting phrase is surrounded by commas:
this rapid orbit, [interrupting phrase], will eventually cause…
Test each option for correct comma placement
Now insert each option and read the middle part:
- A) this rapid orbit, scientists believe, will eventually cause …
- B) this rapid orbit, scientists believe will eventually cause …
- C) this rapid orbit, scientists, believe will eventually cause …
- D) this rapid orbit, scientists, believe, will eventually cause …
Only “scientists believe,” (Choice A) gives you a clean, nonessential phrase scientists believe set off by commas on both sides, without adding awkward or extra commas inside the phrase. Therefore, the correct answer is “scientists believe,”.