Database: world_development_indicators
Question: What is the average number of passengers carried via air transport per year by Bulgaria between 1970 to 1980? Indicate the country's system of trade.
SQL: SELECT AVG(T1.Value), T2.SystemOfTrade FROM Indicators AS T1 INNER JOIN Country AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IndicatorName = 'Air transport, passengers carried' AND T1.Year >= 1970 AND T1.Year < 1981 AND T1.CountryName = 'Bulgaria'