Read this and answer the question. If the question is unanswerable, say "unanswerable".

It is very rare for top clubs to miss the competition, although it can happen in exceptional circumstances. Defending holders Manchester United did not enter the 1999–2000 FA Cup, as they were already in the inaugural Club World Championship, with the club stating that entering both tournaments would overload their fixture schedule and make it more difficult to defend their Champions League and Premiership titles. The club claimed that they did not want to devalue the FA Cup by fielding a weaker side. The move benefited United as they received a two-week break and won the 1999–2000 league title by an 18-point margin, although they did not progress past the group stage of the Club World Championship. The withdrawal from the FA Cup, however, drew considerable criticism as this weakened the tournament's prestige and Sir Alex Ferguson later admitted his regret regarding their handling of the situation.

What FA Cup season did Manchester United enter?
unanswerable