A punctuation question with the same transition in every choice tests sentence breaks and commas. Cover the transition, then test whether each side can stand alone. A semicolon can join two full sentences; a comma alone or no punctuation can't. Then check where the transition belongs: use a comma after an opening transition, before a closing transition, and on both sides of an interruption. Let the ideas on either side guide where you place it.
Hints
- Hint 1
Cover by contrast and look for the subject and main verb on each side. On the left, read the whole thought about what the agronomists found, not only the part beginning with although.
- Hint 2
Check whether the words after the blank can stand alone. What job does chopped into a compact mat do, and where is the main verb?
- Hint 3
Compare the ideas on each side of the blank. How does the report about chopped residue relate to what the passage says about upright residue?
Step-by-step
Cover, split, and place the transition
Step 1Find the first full sentence
Cover by contrast and insert allowed air to circulate, the words shared by every choice. Can the left side stand alone despite although residue left upright cooled the soil? Yes: the agronomists are the subject, who the sentence is about, and found is the main verb, what they did. The words about air circulating finish what they found, so the whole left side is complete.
- Step 2
Test the words after the transition
On the right, can residue chopped into a compact mat restricted airflow stand alone? The subject is residue, the main verb is restricted, and chopped into a compact mat describes the residue. That gives a second full sentence, so the version with commas around by contrast leaves a comma splice, two complete sentences joined only by a comma. The version with no mark before by contrast leaves the same error.
- Step 3
Place the contrast
Which sentence does by contrast belong to? It means in comparison with something different, so try it at the start of the second sentence. Upright residue allowed air to circulate, while the compact mat restricted airflow. The second sentence gives the contrast, so the transition belongs at its start.
- Step 4
Finish the punctuation
A semicolon can join these two full sentences, and by contrast needs a comma after it when it opens the second sentence. The version with no comma after by contrast misses that second mark. The comma after the transition completes the punctuation, giving allowed air to circulate; by contrast,. Choice D.