context: The NASL first staged an indoor tournament in 1971 NASL Professional Hoc-Soc Tournament at the St. Louis Arena with a $2,800 purse. After a couple of years of experimenting, including a three-city tour by the CSKA Moscow team from Moscow in 1974, the league again staged tournaments in 1975 NASL Indoor tournament and 1976 NASL Indoor tournament. For many years Tampa Bay owner George W. Strawbridge, Jr. lobbied his fellow owners to start up a winter indoor season, but was always stone-walled. For several years, his Rowdies and several other teams used winter indoor "friendlies" as part of their training and build-up to the outdoor season. In the meantime, pressed by the rival Major Indoor Soccer League (1978-92) (MISL), which inaugurated play in 1978, two-day mini-tournaments like the 1978 NASL Skelly Indoor Invitational and the 1979 NASL Budweiser Indoor Soccer Invitational were held with varying degrees of success. The NASL finally started a full indoor league schedule, a 12-game season with 10 teams, in 1979-80 NASL Indoor season. For the 1980-81 NASL Indoor season, the number of teams playing indoor soccer increased to 19 and the schedule grew to 18 games. The schedule remained at 18 games, but the teams participating decreased to 13 for the 1981-82 NASL Indoor season. The league canceled the 1982-83 indoor season and three teams (Chicago, Golden Bay, and San Diego) played in the Major Indoor Soccer League (1978-92) for that season. Four other teams (Fort Lauderdale, Montreal, Tampa Bay and Tulsa) competed in a short 1983 NASL Grand Prix of Indoor Soccer in early 1983. The NASL indoor season returned for 1983-84 NASL Indoor season with only seven teams but a 32-game schedule.
question: Which two years in a row in the 1970s were NASL indoor tournaments held?
answer: 1975