FINALE WATCH: 30 Rock (NBC at 10 p.m.) just completed 100 episodes and will march forward into Season 6. In the last episode of Season 5, Liz heads to the Hamptons for some much-needed vacation time but gets an unfortunate next-door neighbor — wild and crazy Tracy Jordan. Meanwhile, Jack seeks comfort while his wife is in captivity and decides Kenneth is the perfect replacement.

Jennifer Lopez pulls double duty on “American Idol” (Fox at 8) on Thursday night — not only is she a judge who has to make a shocked-and-sad face when another singer gets booted, but she fills the role as a results-show entertainer as she performs a song with rapper Pitbull. Plus, country trio Lady Antebellum sing their latest tune before the field is narrowed to the top four singers.

The documentary “Precious Life” (HBO at 8:30) features the story of a 4-month-old Palestinian boy born without an immune system and what happens when he needs a bone marrow transplant procedure available only in an Israeli hospital.

Thursday night, Fox News Channel airs the first Republican presidential debate for the 2012 race (at 9), moderated by anchor Bret Baier in Greenville, S.C.

Now that Steve Carell is gone, Will Ferrell gets The Office (NBC at 9) all to himself — and slowly reveals his character, Deangelo Vickers, to be as crazy, if not crazier, than Michael Scott.

Callie and Arizona get ready for their upcoming wedding onGrey’s Anatomy(ABC at 9), but like many other weddings, the plans aren’t turning out the way they had hoped.

Parker Posey guest-stars as Leslie’s former best friend on “Parks and Recreation” (NBC at 9:30) but doesn’t have kind words for her old BFF when it comes to Pawnee sharing a park with the ritzy neighboring town, Eagleton.

CSI: Crime Scene Investigation(CBS at 9) marks 250 episodes as a notorious killer kidnaps Langston’s ex-wife, Gloria, and leaves Langston a scary message in the process.

New series “Pawn Queens” (TLC at 10, 10:30), which follows owners Minda and Nikki as they run a women-centric pawnshop, features customers in the store looking for everything from vintage Barbie dolls to jewelry.

Actress Jodie Foster visits “Late Show With David Letterman” (CBS at 11:35), along with comedian Tom Dreesen and musical guests Neon Trees.

“The Tonight Show With Jay Leno” (NBC at 11:35) hosts comedian Wanda Sykes, “Dancing With the Stars” contestant/actor Ralph Macchio and singer Priscilla Ahn.

— Emily Yahr