A set of football matches is to be organized in a "round-robin" fashion, i.e., every participating team plays a match against every other team once and only once. If 66 matches are totally played, how many teams participated?

In a round-robin tournament, each team plays every other team exactly once. The total number of matches is given by the formula n(n-1)/2, where n is the number of teams. Setting this equal to 66 gives n(n-1) = 132, which factors to 12 × 11 = 132, so n = 12 teams. Option A (11 teams) would give only 55 matches, while options C and D would give 78 and 91 matches respectively.