A punctuation question with the same words in every choice tests how the sentence fits together. Look around the blank for an insert, a piece of extra information, and find where the main sentence picks up again. Then cross out the insert to check that a complete sentence remains. An insert in the middle needs matching marks at both ends: commas, dashes, or parentheses. A pause alone doesn't decide which punctuation mark belongs.
Hints
- Hint 1
Look back from the blank for where the description of the classification system begins. What punctuation marks that starting point?
- Hint 2
Cross out the description beginning with “an index” and read what remains. How do the words after the blank connect to the meteorologists' use of the system?
- Hint 3
Compare the marks at both ends of the extra information. What does the opening mark tell you about the closing mark?
Step-by-step
Find the insert and match its marks
Step 1Find the opening mark
The dash in system—an index begins an insert, a piece of extra information about the Köppen system. We need to find where it ends to choose the punctuation. What is the main sentence saying? Meteorologists use the system; the index description tells us what that system is. The main verb is use, while categorizes belongs to the description of the index.
- Step 2
Find where the sentence picks up
Where does the index description end? It explains how regions are grouped, and the final item in that explanation is precipitation. After the blank, the sentence returns to why meteorologists use the system: to compare climatic data across continents. If we cross out the whole index description, we still have a complete sentence saying that meteorologists use the system to compare climate data, so the description ends before to compare.
- Step 3
Close the insert with the same mark
To close the insert opened at system—an index, we need to follow the rule for extra information in the middle of a sentence: the closing mark must match the opening mark. A comma would create a mixed pair, and neither a colon nor a semicolon can close an insert opened by a dash. The version precipitation—to adds a closing dash and lets the sentence continue with to compare. Choice B.