Question 23·Medium·Boundaries
To trace changes in Earth's magnetic field over geological time, scientists rely on _____ the study of magnetic signals preserved in rocks—as an indirect means of observation.
Which choice completes the text so that it conforms to the conventions of Standard English?
For punctuation and boundary questions, first find the core sentence by mentally removing any interrupting phrase; make sure the remaining sentence is complete and logical. Then identify whether the middle phrase is extra (nonessential) information—if so, it should be set off by matching punctuation marks (commas, parentheses, or dashes). Finally, check each choice for consistency: avoid mixing different punctuation types around the same interrupting phrase, and make sure you do not create fragments or run-ons when you plug the option back into the sentence.
Hints
Find the main sentence without the middle phrase
Try reading the sentence while skipping from the word in the blank directly to "as an indirect means of observation." What is the core idea?
Decide what "the study of magnetic signals preserved in rocks" is doing
Ask yourself: Is this phrase essential to the sentence, or is it extra information that explains the word in the blank?
Match the punctuation marks
Notice there is already a dash before "as an indirect means of observation." If the middle phrase is being set off, what kind of punctuation mark do you need before "the" to match that existing dash?
Step-by-step Explanation
See the core sentence
First, strip out the extra descriptive phrase to see the main structure:
"To trace changes in Earth's magnetic field over geological time, scientists rely on paleomagnetism … as an indirect means of observation."
This shows that "paleomagnetism" is the main object, and the rest of the sentence still makes sense without the middle phrase.
Identify the interrupting phrase
Now look at the middle part:
"the study of magnetic signals preserved in rocks—"
This defines or explains "paleomagnetism," so it is an appositive (a phrase that renames or explains a noun). Appositives that interrupt a sentence are usually set off by matching punctuation marks: commas, parentheses, or dashes.
Notice the existing punctuation
There is already an em dash before "as":
"... the study of magnetic signals preserved in rocks—as an indirect means of observation."
That dash is closing off the interrupting definition. So, whatever punctuation you choose before "the" must match this em dash, forming a pair around the explanatory phrase.
Check each option for matching punctuation
We need a mark before "the" that:
- Sets off the explanatory phrase, and
- Matches the em dash already in the sentence.
Choices that add a comma, parenthesis, or colon will not match the existing dash and will create inconsistent or incorrect punctuation. The only choice that supplies a second em dash, making a correct pair around the explanatory phrase, is "paleomagnetism—the", so that is the correct answer.