Which sentence uses a nonrestrictive clause with commas correctly?

• A. The lab, which was new, failed.
• B. The lab which was new, failed.
• C. The lab, which was new failed.
• D. The lab which was new failed.

Answer: (A)

Nonrestrictive clauses add extra information about a noun and need to be set off from the rest of the sentence with commas. Here, “which was new” gives additional detail about the lab but isn’t needed to identify which lab is being talked about, so it should be enclosed by two commas. That gives The lab, which was new, failed, which is the correct punctuation pattern for a nonrestrictive clause.

The other forms misplace or omit punctuation around the clause, making the phrase either unnecessarily restrictive or simply ungrammatical in standard usage. For example, leaving out the closing comma around the nonrestrictive clause or placing the comma in the wrong spot breaks the intended meaning and rhythm. Using a restrictive feel with which (without proper commas) also changes how the clause functions, which isn’t appropriate here.