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Association football or soccer, remains one of the most popular games played in the British Isles. Every Saturday from late August until the beginning of May, large crowds of people support their sides in football grounds up and down the country, while an almost equally large number of people play the game in club teams of every imaginable variety and level of skill. Over the last twenty years, though, the attendance at football matches has fallen away sharply. This is because of changing lifestyles and football hooligans. As violence at and near the football grounds increased, there was an ever-increasing tendency for people to stay away, leaving the grounds to football fans.
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The attendance at football matches has fallen away sharply ________________________.
over the last fifty years
over the last twenty-one years
over the last thirty years
over the last twenty years