Question 125·Medium·Boundaries
A groundbreaking study published in Marine Biology found that reef fish exhibiting ultraviolet patterns—_____ can communicate covertly without alerting predators.
Which choice completes the text so that it conforms to the conventions of Standard English?
For SAT punctuation and boundary questions, first read the sentence around the blank and identify the structure: is the missing part an interrupter, a list item, or part of the main clause? Check what punctuation is already present before and after the blank, then choose the option that matches and correctly closes that structure (for example, pairing commas with commas or dashes with dashes). Finally, read the sentence with your chosen option to ensure it flows naturally and does not introduce unnecessary commas or sentence fragments.
Hints
Notice the punctuation already in the sentence
Look at the punctuation immediately before the blank and immediately after it. What mark is already used before the blank, and how does the sentence continue afterward?
Think about interrupter phrases
The words you add will give extra information about "ultraviolet patterns." Should that extra information be fully part of the main clause, or set off as a nonessential (interrupting) phrase?
Match the punctuation style
If the sentence starts an interruption with a particular punctuation mark, like a dash, what should you use to close that interruption before the main clause resumes?
Check for unnecessary commas
Read each option inside the sentence. Does any comma split a noun from the words that describe it in a way that sounds choppy or unnecessary?
Step-by-step Explanation
Identify the structure of the sentence
Read the full sentence with the blank in mind:
"A groundbreaking study published in Marine Biology found that reef fish exhibiting ultraviolet patterns—_____ can communicate covertly without alerting predators."
Notice that there is already an em dash just before the blank, and then the main clause continues with "can communicate..." after the blank. This suggests the blank holds an interrupting phrase that should be set off by a pair of matching punctuation marks (here, em dashes).
Recognize the type of phrase in the blank
The phrase that will go in the blank is describing or explaining the "ultraviolet patterns" that the reef fish exhibit. That kind of extra information is called a nonessential or parenthetical phrase. In this sentence, it interrupts the main idea and should be enclosed by matching punctuation—either commas, parentheses, or, as already signaled, em dashes.
Match the punctuation already started
Because there is already an em dash before the blank, the phrase in the blank must end with another em dash to close off the interruption before the main clause continues with "can communicate covertly..." Any choice that does not provide that closing em dash is incorrect, because it would leave the dash interruption open or change the punctuation style halfway through.
Avoid unnecessary commas inside the phrase
Within the interrupting phrase, the words should read smoothly: "details invisible to most predators" is a single noun phrase. Placing a comma after "details" would wrongly split the noun from its describing words and create a pause that is not needed. So the correct choice must:
- include a closing em dash at the end of the phrase, and
- not add a comma after "details".
The option that does exactly this is "details invisible to most predators—", which fits neatly between the dashes and keeps the phrase grammatically correct.