A punctuation question with a description inside the sentence tests whether you can set off extra information. Find the subject and its main verb, then locate the description between them. Cross out the description to see whether a complete sentence remains. Commas or dashes can come in pairs around extra information; a semicolon joins two complete sentences without a linking word. Then check both ends, because a mark can look right alone but fail as part of a pair.
Hints
- Hint 1
Look at the verb after the blank. Which noun is doing the revealing?
- Hint 2
Read the sentence without the time description, then put it back. Check how the description connects to the rest of the sentence.
Step-by-step
Find the description and match its marks
Step 1Find what the images do
The words reveal startling changes tell us what the satellite images do, so start there to work out the punctuation at the blank. What is the subject, the noun doing the revealing? It's images, and reveal is the main verb, the verb that tells what the subject does. The images need to stay connected to their main verb even with a description between them.
- Step 2
Cross out the time description
What does taken over a five-year period add to the sentence? It tells us when the images were taken, and taken describes images rather than being a main verb because there's no helper such as were. Cross out the phrase and its marks: The satellite images reveal startling changes in the glacier's size. That complete sentence shows that the time description is extra information the sentence can do without.
- Step 3
Match the two ends
Now compare the marks around the description before reveal: extra information in the middle needs matching marks on its two sides. The version with a dash then a comma and the version with a comma then a dash mix marks, while the semicolon version cuts the subject off from its main verb. The version with commas on both sides of taken over a five-year period gives the description a matching pair. Choice A.