Which version correctly punctuates the nonrestrictive clause in the sentence about 30 Rock?

• A. The characters and plot lines of 30 Rock as most of the show's fans know grew out of Tina Fey's experiences at Saturday Night Live.
• B. The characters and plot lines of 30 Rock as most of the show's fans know, grew out of Tina Fey's experiences at Saturday Night Live.
• C. The characters and plot lines of 30 Rock, as most of the show's fans know grew out of Tina Fey's experiences at Saturday Night Live.
• D. The characters and plot lines of 30 Rock, as most of the show's fans know, grew out of Tina Fey's experiences at Saturday Night Live.

Answer: (D)

Nonrestrictive, or parenthetical, clauses add information that isn’t essential to identifying the subject and must be set off by commas. In this sentence, the phrase as most of the show's fans know functions as an aside—it's extra information about what fans know, not essential to which characters or plot lines are being talked about. So it should be enclosed by commas on both sides: The characters and plot lines of 30 Rock, as most of the show's fans know, grew out of Tina Fey's experiences at Saturday Night Live.

That’s why the version with comma placement on both sides is correct. Without the opening comma, or without the closing comma, the interrupter isn’t properly set off, which can make the meaning feel restrictive or cause the sentence to read incorrectly.