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?
Answer: 2015 GP2 Series