Question: What FIFA World Cup did the referee of the 28 May 2011 football match at Wembley Stadium participate in? 
Facts:
2011 UEFA Champions League Final was a football match at Wembley Stadium played on 28 May 2011
Viktor Kassai refereed the 2011 UEFA Champions League Final
Viktor Kassai refereed the 2010 FIFA World Cup
Answer: 2010 FIFA World Cup