Coming off their win over the Titans, the Broncos remained on home ground for an AFC West rematch with the San Diego Chargers on Thursday Night Football. A 15-yard touchdown pass from quarterback Peyton Manning to wide receiver Andre Caldwell gave the Broncos the early lead. The Chargers subsequently got on the board, with a 38-yard field goal by placekicker Nick Novak. The Broncos answered, with a 32-yard field goal by placekicker Matt Prater. However, the Chargers would score 21 unanswered points and dominate the time of possession, as the Broncos' offense went three-and-out on all three of their second quarter possessions. A 19-yard touchdown pass from quarterback Philip Rivers to wide receiver Keenan Allen tied the game, and the Chargers grabbed the lead late in the first half, with a 10-yard touchdown pass from Rivers to Allen. The Chargers took the opening possession of the second half and increased their lead, culminating with running back Ryan Mathews rushing for a 23-yard touchdown. The Broncos were forced to punt after only four plays on their initial possession of the second half, and the Chargers' ball control offense subsequently ate up over eight minutes of the third quarter, though the Broncos' defense forced a Chargers' punt. The Broncos cut into the Chargers' lead in the fourth quarter, with a 5-yard touchdown pass from Manning to Caldwell with 10:30 remaining in the game. After forcing a Chargers' punt, the Broncos offense got the football with 5:50 remaining, however, three plays later, Manning was intercepted by Chargers' linebacker Thomas Keiser at the Broncos' 39-yard line. Six plays later, a 35-yard field goal by Novak increased the Chargers' lead to 27-17 with 2:41 remaining. With no timeouts remaining (save the two-minute warning), the Broncos tried to rally, with Prater kicking a 42-yard field goal with only 34 seconds remaining. However, the Broncos' onside kick attempt was unsuccessful, sealing the win for the Chargers. With the loss, the Broncos had their 13-game regular-season home winning streak snapped, as well as their 10-game winning streak against division opponents.

Answer this question based on the article: How many touchdown passes did Rivers throw in the second quarter?
2