Question: The 2015 Bahrain GP2 Series round was a pair of motor races held on 18 and 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain as part of the GP2 Series, it was the first round of which series that was the eleventh GP2 Series season, a championship for open-wheel racing cars run as a support series to the 2015 Formula One World Championship?
Context: The 2015 Bahrain GP2 Series round was a pair of motor races held on 18 and 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain as part of the GP2 Series.  It was the first round of the 2015 GP2 Series and was run in support of the 2015 Bahrain Grand Prix.  The first race, a 32-lap feature event, was won by ART Grand Prix driver Stoffel Vandoorne who started from the pole position.  Rio Haryanto finished second for the Campos Racing team, and Racing Engineering driver Alexander Rossi came third.  Haryanto won the second event, a 23-lap sprint race, ahead of Vandoorne in second, and Lazarus driver Nathanaël Berthon in third. The 2012 Monaco GP2 Series round and the 2012 Monaco GP3 Series round will be a group of motor races held at the Circuit de Monaco in the Principality of Monaco for the GP2 and GP3 Series championships The races, held on 25 and 26 May, will be in support of the Monaco Formula One Grand Prix.  The GP2 races will be the fifth round of the 2012 GP2 championship, while the GP3 races will form the second round of the 2012 GP3 season.  2012 marks the first time that the GP3 Series will hold a race at the Circuit de Monaco. The 2012 Bahrain GP2 Series rounds were a series of GP2 Series motor races held at the Bahrain International Circuit as part of the 2012 GP2 Series season.  The first round was held on 21 and 22 April, in support of the 2012 Bahrain Grand Prix.  The second round was held one week later as an independent round of the championship, the only one of its kind on the 2012 calendar. The 2012 Monza GP2 series round and the 2012 Monza GP3 Series round were a group of motor races held at the Autodromo Nazionale Monza near Monza in Italy for the GP2 and GP3 Series championships.  The races, which were run on 8 and 9 September, was in support of the Italian Formula One Grand Prix.  The GP2 races formed the eleventh round of the 2012 GP2 championship, while the GP3 races were the eighth and final round of the 2012 GP3 season. The 2012 Marina Bay GP2 Series round is a group of motor races that will be held at the Marina Bay Street Circuit in Marina Bay, Singapore for the GP2 Series championship, with the 2012 GP3 Series season having concluded at the previous round at Monza.  The races, which will be held on 22 and 23 July, will be in support of the Singapore Formula One Grand Prix, and represent the final round of the 2012 GP2 Series season.  The event marks the first time the GP2 Series has travelled to Singapore. The 2014 Bahrain GP2 Series round was a pair of motor races held on 6 and 7 April 2014 at the Bahrain International Circuit in Sakhir, Bahrain as part of the GP2 Series.  It was the first round of the 2014 GP2 Series and was run in support of the 2014 Bahrain Grand Prix.  The first race, a 32-lap feature event, was won by ART Grand Prix driver Stoffel Vandoorne after starting from second position.  Julián Leal finished second for the Carlin team and DAMS driver Jolyon Palmer came in third.  Palmer won the second race, a 23-lap sprint event, ahead of Rapax driver Simon Trummer in second and Leal third. The 2015 GP2 Series season was the eleventh GP2 Series season, a championship for open-wheel racing cars run as a support series to the 2015 Formula One World Championship.  Twenty-six drivers representing thirteen teams raced over eleven rounds, starting in Bahrain on 18 April and finishing in Abu Dhabi on 29 November. The 2015 Bahrain Grand Prix (formally known as the 2015 Formula 1 Gulf Air Bahrain Grand Prix) was a Formula One motor race that was held on 19 April 2015 at the Bahrain International Circuit in Sakhir, Bahrain.  The race was the fourth round of the 2015 season and marked the eleventh time that the Bahrain Grand Prix has been run as a round of the Formula One World Championship. The 2013 Bahrain GP2 Series round is a pair of motor races held on March 23 and 24, 2013 at Sepang International Circuit, Malaysia as part of the GP2 Series.  It is the second round of the 2013 season.  The race weekend supported the 2013 Bahrain Grand Prix. The 2005 Bahrain GP2 Series round was a GP2 Series motor race held on September 29 and 30, 2005 at the Bahrain International Circuit in Sakhir, Bahrain.  It was the final round of the 2005 GP2 Series season.  The race weekend was a stand-alone event unlike the previous GP2 rounds which support Formula One Grands Prix.
Answer: 2015 GP2 Series