During the guided tour, the naturalist pointed out the river otter— a semiaquatic mammal native to _____ as it slid playfully down the muddy riverbank.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation-boundary questions, first strip out any descriptive middle phrase to see the core sentence and check that it reads smoothly. Then identify how that extra phrase is introduced (comma, dash, parentheses) and make sure it is closed with the same type of punctuation. Finally, test each option by reading the full sentence in your head and reject choices that either change the sentence structure (like forcing a semicolon between non–complete clauses) or create mismatched punctuation around the interrupting phrase.
Hints
Find the main sentence without the middle phrase
Try reading the sentence without the words between the blank and the dash before “a semiaquatic.” Does it still make sense? That will show you where the extra information begins and ends.
Notice the punctuation before the extra information
Look at the mark right before the words “a semiaquatic mammal native to North America.” What punctuation is being used to introduce that descriptive phrase?
Think about matching punctuation
When a descriptive phrase interrupts a sentence, English usually uses a matching pair of punctuation marks to set it off. Ask yourself: what mark is already used to open the phrase, and which option would correctly close it?
Step-by-step Explanation
Identify the extra information (interrupting phrase)
Find the part of the sentence that interrupts the main idea:
“the naturalist pointed out the river otter— a semiaquatic mammal native to North America _____ as it slid playfully down the muddy riverbank.”
The bold part is extra information describing “the river otter.” If you remove it, the sentence still makes sense:
“During the guided tour, the naturalist pointed out the river otter as it slid playfully down the muddy riverbank.”
So this is a nonessential (extra) phrase that must be set off correctly with punctuation.
Match how the phrase is opened and closed
Look at how the extra phrase starts: it is introduced by an em dash right before “a semiaquatic.”
In English, when you use punctuation to set off an interrupting phrase, you must match the punctuation marks:
- commas pair with commas
- parentheses pair with parentheses
- dashes pair with dashes
So the punctuation after “North America” must match the em dash that appears before the phrase.
Eliminate options that break the sentence structure
Check each answer choice and think about how it affects the structure:
- A comma after “North America” would give a dash–comma pair, which is inconsistent.
- A semicolon would suggest a second independent clause after it, but “as it slid playfully down the muddy riverbank” is not an independent clause.
- A colon normally follows a complete sentence to introduce a list or explanation, which does not fit here and would also not match the starting dash.
The only option that correctly closes the interrupting phrase and matches the opening punctuation mark is the em dash, so the correct completion is “North America—” (choice D).