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)

A nonrestrictive clause adds extra information about something already identified and is set off by commas. The signal here is using which with two commas to show that the extra detail about the duration is incidental to which experiment is being talked about. The clause can be removed without changing which experiment is intended, so the correct punctuation uses a comma before the clause and a comma after it: The experiment, which lasted two hours, failed.

The other options misplace the punctuation or use the pronoun in a way that makes the clause seem essential (restrictive) rather than optional. When the clause isn’t properly enclosed by commas, or when the structure suggests the information is essential to identify the experiment, the sentence doesn’t fit the nonrestrictive pattern.