Question 116·Medium·Boundaries
Many meteorologists use the Köppen climate classification system—an index that categorizes regions according to average temperature and _____ compare climatic data across continents.
Which choice completes the text so that it conforms to the conventions of Standard English?
For boundary questions, first identify whether the underlined/blank area ends an interrupting (nonessential) phrase. If the phrase is opened with a comma, dash, or parenthesis, it should generally be closed with the same punctuation. Then eliminate colons and semicolons unless the surrounding text forms the specific structures they require (colon after an independent clause; semicolon between independent clauses).
Hints
Find the core sentence
Read the sentence without the phrase “an index that categorizes regions according to average temperature and precipitation.” Does the remaining sentence make sense and have a clear main structure?
Notice the punctuation that starts the interruption
Right before “an index…,” the sentence uses an em dash. What punctuation mark is typically used to close an interruption that was opened with an em dash?
Eliminate punctuation that doesn’t fit the structure
After the blank, the sentence continues with “compare climatic data…,” which is not an independent clause. Which punctuation mark choice would be inappropriate for that structure, and which choice correctly matches the opening punctuation?
Step-by-step Explanation
Identify the main sentence and the extra information
Ignore the middle phrase for a moment and read the core sentence:
“Many meteorologists use the Köppen climate classification system … to compare climatic data across continents.”
The middle part—“an index that categorizes regions according to average temperature and precipitation”—is extra information explaining what the Köppen system is. This kind of extra information is a nonessential (interrupting) phrase.
Use matching punctuation to set off the interruption
The sentence already uses an em dash to introduce the interrupting phrase:
“…the Köppen climate classification system—an index that categorizes regions according to average temperature and ____ compare …”
When a nonessential phrase is inserted mid-sentence, it must be set off with matching punctuation on both sides (comma/comma, dash/dash, or parentheses/parentheses).
Select the option that correctly closes the interrupting phrase
Because the interruption begins with a dash, it must end with a dash as well. The only option that closes the interrupting phrase and allows the main sentence to continue smoothly is precipitation—to.