A punctuation question with the same word in every choice tests how the words on either side connect. Find the blank's job by reading both sides and asking which can stand alone as a sentence. Two full sentences need punctuation; a comma alone can't join them, and and needs a comma before it. A colon can introduce an explanation after a full sentence. Use sentence structure rather than pauses, since a natural pause doesn't tell you which mark belongs.
Hints
- Hint 1
Read “They face a persistent” with the word all the choices share. Test whether that part can stand alone as a sentence.
- Hint 2
Look after the blank for a subject and main verb, and read through “after storms.” How do those words relate to the difficulty the ecologists face?
Step-by-step
Test both sides of the blank
Step 1Check the opening statement
Read They face a persistent with problem, the word all the choices share. Can those words stand alone? They tell us who has a problem and what they face, so the words before the punctuation form a full sentence. We're choosing how to attach the rest, not changing what that opening says.
- Step 2
Read through the words after the blank
After the blank, a colony's shape is the subject and can change is the main verb. Does the phrase making comparisons between older and newer photographs difficult change that? No: it adds the result of the shape change, so the words after the blank can still stand alone. Two full sentences meet here, so the version with no mark runs them together. The version with a comma alone creates a comma splice, an error that joins full sentences with only a comma.
- Step 3
Use the link between the sentences
The version with and still joins two full sentences without the comma needed before and. How do the sentences connect? The second explains the problem: a change in shape is making comparisons between older and newer photographs difficult. A colon can follow a full sentence to introduce an explanation, so it connects the problem with the words that explain it. Choice B.