Context: Hoping to rebound from their last-second home loss to the Panthers, the Chargers flew to Invesco Field at Mile High for a Week 2 AFC West duel with the Denver Broncos.  In the first quarter, San Diego trailed early as FB Michael Pittman getting a 1-yard TD run, after a Philip Rivers pass to Chris Chambers was ruled an interception on the field. The Chargers challenged the ruling, but the replay system was inoperative to the officials, so the ruling on the field stood.  The 'Bolts would respond with kicker Nate Kaeding getting a 34-yard field goal.  In the second quarter, Denver increased its lead with QB Jay Cutler completing a 3-yard and a 14-yard TD pass to TE Tony Scheffler.  San Diego would immediately respond with RB Darren Sproles returning a kickoff 103 yard for a touchdown, yet the Broncos replied with kicker Matt Prater getting a 52-yard field goal.  The Chargers would strike at Denver's lead as QB Philip Rivers completing a 48-yard TD pass to WR Chris Chambers, yet the Broncos would end the half with Cutler completing a 6-yard TD pass to WR Brandon Marshall. In the third quarter, the 'Bolts began to rally as Rivers completed a 15-yard TD pass to Chambers, along with Kaeding nailing a 21-yard field goal.  In the fourth quarter, San Diego took the lead with Kaeding's 28-yard field goal and Rivers' 66-yard TD pass to Sproles.  However, Denver responded with a 12-play, 80-yard drive (which included a second controversial officiating call that turned a game-winning fumble into an incomplete pass) that concluded with Cutler completing a 4-yard TD pass to WR Eddie Royal, including Cutler's 2-point conversion pass to Royal. Notable: The game has been titled by press and websites as "The Mile High Highjacking" after the officiating call that cost San Diego the game.

Question: What ruling did the Chargers challenge in the first quarter?

Answer:
interception