Which sentence introduces a nonrestrictive clause with correct punctuation?

Which sentence introduces a nonrestrictive clause with correct punctuation?

• A. The experiment, which lasted two hours, failed.
• B. The experiment which lasted two hours, failed.
• C. The experiment, which lasted two hours failed.
• D. The experiment which lasted two hours, failed.

Answer: (A)

Nonrestrictive clauses add extra information and are set off by commas. Here, “which lasted two hours” gives additional detail about the experiment but isn’t needed to identify which experiment is being talked about. So it should be enclosed by two commas, signaling that the clause is optional to the main point: The experiment, which lasted two hours, failed.

The other forms misplace or omit the punctuation around the clause, making the information either essential to identify the subject (restrictive) or awkward in how the sentence flows.