The TD Garden, formerly called the FleetCenter and built to replace the old, since-demolished Boston Garden, is adjoined to North Station and is the home of two major league teams: the Boston Bruins of the National Hockey League and the Boston Celtics of the National Basketball Association. The arena seats 18,624 for basketball games and 17,565 for ice hockey games. The Bruins were the first American member of the National Hockey League and an Original Six franchise. The Boston Celtics were founding members of the Basketball Association of America, one of the two leagues that merged to form the NBA. The Celtics have the distinction of having won more championships than any other NBA team, with seventeen.

Answer this question, if possible (if impossible, reply "unanswerable"): What was the FleetCenter built to replace?
Boston Garden