In reviewing conflicting no-till wheat trials, agronomists found that although residue left upright cooled the soil, it still _____ residue chopped into a compact mat restricted airflow and delayed spring warming.
Which choice completes the text so that it conforms to the conventions of Standard English?
When an option contains a transition between two clauses, evaluate two punctuation decisions separately: first determine whether the clauses need a comma, semicolon, or other boundary; then check whether the transition itself must be set off with a comma. Do not choose an option merely because one of its punctuation marks is correct.
Hints
Find the sentence units
Determine whether the material on each side of the transition can stand as a complete sentence.
Check the clause boundary
Consider what punctuation can join two closely related complete clauses without creating a comma splice.
Check both sides of the transition
A transitional phrase between clauses may require punctuation both before and after it. Evaluate every mark included in each option.
Step-by-step Explanation
Identify the first complete clause
The first sentence unit is complete: “although residue left upright cooled the soil, it still allowed air to circulate.” The introductory dependent clause beginning with “although” is followed by the main clause beginning with “it.”
Identify the second complete clause
The words “residue chopped into a compact mat restricted airflow and delayed spring warming” also form a complete clause, with “residue” as the subject and “restricted” and “delayed” as the verbs.
Punctuate the transition
The transitional phrase “by contrast” links the two complete clauses but cannot join them with a comma alone. A semicolon is needed before the transition, and a comma is needed after it: “allowed air to circulate; by contrast, residue chopped into a compact mat restricted airflow and delayed spring warming.”