Write SQL query to solve given problem: 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.. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'