Roy Perkins (R) of the United States prepares prior to the Men's 50m Freestyle S5 Heats of London Paralympic Games in London, Britain, Aug. 30, 2012. The swimming event of London Paralympic Games kicked off on Thursday. (Xinhua/Zhang Chuanqi)(Xinhua/Zhang Chuanqi)

 Jonas Larsen of Denmark prepares prior to the Men's 50m Freestyle S5 Heats of London Paralympic Games in London, Britain, Aug. 30, 2012. The swimming event of London Paralympic Games kicked off on Thursday. (Xinhua/Zhang Chuanqi)

Zsolt Vereczkei of Hungary competes during the Men's 50m Freestyle S5 Heats of London Paralympic Games in London, Britain, Aug. 30, 2012. The swimming event of London Paralympic Games kicked off on Thursday. (Xinhua/Zhang Chuanqi)

Anthony Stephens of Britain competes during the Men's 50m Freestyle S5 Heats of London Paralympic Games in London, Britain, Aug. 30, 2012. The swimming event of London Paralympic Games kicked off on Thursday. (Xinhua/Zhang Chuanqi)

A boy watches the swimming match of the London Paralympic Games in London, Britain, Aug. 30, 2012. The swimming event of London Paralympic Games kicked off on Thursday. (Xinhua/Zhang Chuanqi)