China’s Olympic athletes and coaches must vow to reject performance enhancing substances and pass an exam on doping if they want to participate in the London Games, state press said Friday. Athletes have began gathering at Beijing’s national sports training centre where they must take an oath before the nation’s flag and vow to remain clean at the London Olympics, the China Daily reported on its website. “Athletes and coaches are qualified to join the national team when they have passed the exam, vowed to abide by anti-doping, and signed the commitment letter,” Shen Chunde, a top track and field official, said.