Q:Question: "What stops countries from choosing all their best professional players?"  Context: "There has been a football tournament at every Summer Olympic Games since 1900, except at the 1932 games in Los Angeles. Before the inception of the World Cup, the Olympics (especially during the 1920s) had the same status as the World Cup. Originally, the event was for amateurs only; however, since the 1984 Summer Olympics, professional players have been permitted, albeit with certain restrictions which prevent countries from fielding their strongest sides. The Olympic men's tournament is played at Under-23 level. In the past the Olympics have allowed a restricted number of over-age players per team. A women's tournament was added in 1996; in contrast to the men's event, full international sides without age restrictions play the women's Olympic tournament."  Answer:
A:
certain restrictions