Question 35·Medium·Boundaries
Geologists studying volcanic eruptions pay particular attention to _____—the mixture of hot gases and ash that races down a volcano's slopes at tremendous speeds—to predict potential hazards.
Which choice completes the text so that it conforms to the conventions of Standard English?
For SAT punctuation and boundaries questions, first remove any material between commas, dashes, or parentheses to see the core sentence and ensure it is complete. Then, look at the punctuation already provided in the sentence and avoid choices that add redundant or conflicting marks (like adding a comma or colon right before a dash) or that leave words like "the" stranded. Finally, read each remaining option back into the sentence to check for a smooth, grammatical flow.
Hints
Look at the punctuation around the blank
Notice that there is already a dash immediately after the blank, and another dash later on. That structure usually sets off an explanatory phrase in the middle of a sentence.
Strip out the middle and see what’s left
Temporarily ignore the phrase between the dashes ("the mixture of hot gases and ash...") and read the sentence without it. Think about what kind of word or phrase should go in the blank so the main sentence is complete.
Check for extra or doubled punctuation/words
Ask yourself: since the dash is already there to introduce the explanation, do we need a comma, colon, or parenthesis right before it? Also, the phrase after the dash already begins with "the"—do we need another "the" just before the dash?
Step-by-step Explanation
Identify the structure of the sentence
Focus on the overall structure:
"Geologists studying volcanic eruptions pay particular attention to __ — the mixture of hot gases and ash that races down a volcano's slopes at tremendous speeds — to predict potential hazards."
The blank is part of the phrase "pay particular attention to __" and is followed immediately by a dash. The part between the dashes is an explanation of whatever fills the blank.
Recognize what kind of word/phrase is needed
If you temporarily remove the explanatory part between the dashes, the core sentence is:
"Geologists studying volcanic eruptions pay particular attention to __ to predict potential hazards."
So the blank must be a noun or noun phrase (the thing geologists pay attention to), not extra punctuation or a fragment. The dash after the blank already introduces the explanation that starts with "the mixture of hot gases and ash..."
Consider the role of the dash and nearby words
A dash already signals that an explanation or definition is coming. That means:
- You do not need a comma, colon, or parenthesis right before the dash.
- The phrase after the dash already starts with the word "the" ("the mixture of hot gases and ash..."). Adding another "the" right before the dash will create an awkward and incorrect structure like "..., the—the mixture..." or similar.
So any choice that adds extra punctuation or an extra "the" right before the dash will violate standard English punctuation and sentence flow.
Test the choices and select the one that fits cleanly
Now read the sentence with each option:
- With "pyroclastic flows, the": "...pay particular attention to pyroclastic flows, the—the mixture of hot gases..." This creates a stray "the" and an unnecessary comma before the dash.
- With "pyroclastic flows: the": "...pay particular attention to pyroclastic flows: the—the mixture of hot gases..." This piles a colon and a dash together and again leaves a stray "the."
- With "pyroclastic flows (the": "...pay particular attention to pyroclastic flows (the—the mixture of hot gases..." This starts a parenthesis that is never closed and still has the same "the—the" problem.
- With "pyroclastic flows": "...pay particular attention to pyroclastic flows—the mixture of hot gases and ash that races down a volcano's slopes at tremendous speeds—to predict potential hazards."
Only the last version is smooth, grammatical, and correctly punctuated. Therefore, the correct answer is "pyroclastic flows."