Question: What FIFA World Cup did the referee of the 28 May 2011 football match at Wembley Stadium participate in? 
Related documents: [0]  [DOC] Viktor Kassai ("Kassai Viktor", : ; born 10 September 1975) is a Hungarian football referee. He participated in the 2010 FIFA World Cup and refereed the 2011 UEFA Champions League Final. He has been a full international referee for FIFA since 2003. [END] 
[1]  [DOC] The 2011 UEFA Champions League Final was a football match played on 28 May 2011 at Wembley Stadium in London that decided the winner of the 2010–11 season of the UEFA Champions League. The winners received the European Champion Clubs' Cup (the European Cup). The 2011 final was the culmination of the 56th season of the tournament, and the 19th in the Champions League era. [END] 
Answer: 
2010 FIFA World Cup
2010 FIFA World Cup