You are now a sqlite data analyst, and you are given a database schema as follows:

    【Schema】
    【DB_ID】 world_development_indicators
【Schema】
# Table: Series
[
(SeriesCode:TEXT, Primary Key, Examples: [AG.AGR.TRAC.NO, AG.CON.FERT.PT.ZS, AG.CON.FERT.ZS]),
(Topic:TEXT),
(IndicatorName:TEXT),
(ShortDefinition:TEXT),
(LongDefinition:TEXT),
(UnitOfMeasure:TEXT, Examples: [`, %, 2005 PPP $]),
(Periodicity:TEXT, Examples: [Annual]),
(BasePeriod:TEXT, Examples: [2005, varies by country, 2011]),
(OtherNotes:INTEGER),
(AggregationMethod:TEXT, Examples: [Weighted average, Sum, Gap-filled total]),
(LimitationsAndExceptions:TEXT),
(NotesFromOriginalSource:TEXT),
(GeneralComments:TEXT),
(Source:TEXT),
(StatisticalConceptAndMethodology:TEXT),
(DevelopmentRelevance:TEXT),
(RelatedSourceLinks:TEXT),
(OtherWebLinks:INTEGER),
(RelatedIndicators:INTEGER),
(LicenseType:TEXT, Examples: [Open, Restricted])
]
# Table: Footnotes
[
(Countrycode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(Seriescode:TEXT, Primary Key, Examples: [AG.LND.FRST.K2, BX.KLT.DINV.CD.WD, DC.DAC.AUSL.CD]),
(Year:TEXT, Primary Key, Examples: [YR1990, YR2000, YR2005]),
(Description:TEXT)
]
# Table: Indicators
[
(CountryName:TEXT, Examples: [Arab World]),
(CountryCode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(IndicatorName:TEXT),
(IndicatorCode:TEXT, Primary Key, Examples: [AG.LND.AGRI.K2, AG.LND.AGRI.ZS, AG.LND.ARBL.HA]),
(Year:INTEGER, Primary Key, Examples: [1961, 1962, 1963]),
(Value:INTEGER, Examples: [133, 87, 6])
]
# Table: SeriesNotes
[
(Seriescode:TEXT, Primary Key, Examples: [DT.DOD.PVLX.CD, IC.CUS.DURS.EX, IC.FRM.BNKS.ZS]),
(Year:TEXT, Primary Key, Examples: [YR2014, YR2002, YR2003]),
(Description:TEXT)
]
# Table: Country
[
(CountryCode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(ShortName:TEXT, Examples: [Afghanistan, Albania, Algeria]),
(TableName:TEXT, Examples: [Afghanistan, Albania, Algeria]),
(LongName:TEXT, Examples: [Islamic State of Afghanistan]),
(Alpha2Code:TEXT, Examples: [AF, AL, DZ]),
(CurrencyUnit:TEXT, Examples: [Afghan afghani, Albanian lek, Algerian dinar]),
(SpecialNotes:TEXT),
(Region:TEXT, Examples: [South Asia]),
(IncomeGroup:TEXT, Examples: [Low income, Upper middle income, High income: nonOECD]),
(Wb2Code:TEXT, Examples: [AF, AL, DZ]),
(NationalAccountsBaseYear:TEXT, Examples: [2002/03]),
(NationalAccountsReferenceYear:TEXT, Examples: [1996, 2013/14, 2010]),
(SnaPriceValuation:TEXT, Examples: [Value added at basic prices (VAB)]),
(LendingCategory:TEXT, Examples: [IDA, IBRD, Blend]),
(OtherGroups:TEXT, Examples: [HIPC, Euro area]),
(SystemOfNationalAccounts:TEXT),
(AlternativeConversionFactor:TEXT, Examples: [199196, 197184, 199095]),
(PppSurveyYear:TEXT, Examples: [Rolling]),
(BalanceOfPaymentsManualInUse:TEXT, Examples: [IMF Balance of Payments Manual, 6th edition.]),
(ExternalDebtReportingStatus:TEXT, Examples: [Actual, Preliminary, Estimate]),
(SystemOfTrade:TEXT, Examples: [General trade system, Special trade system]),
(GovernmentAccountingConcept:TEXT, Examples: [Consolidated central government]),
(ImfDataDisseminationStandard:TEXT, Examples: [General Data Dissemination System (GDDS)]),
(LatestPopulationCensus:TEXT, Examples: [1979, 2011, 2008]),
(LatestHouseholdSurvey:TEXT, Examples: [Multiple Indicator Cluster Survey (MICS), 2010/11]),
(SourceOfMostRecentIncomeAndExpenditureData:TEXT),
(VitalRegistrationComplete:TEXT, Examples: [Yes]),
(LatestAgriculturalCensus:TEXT, Examples: [2013/14, 2012, 2007]),
(LatestIndustrialData:INTEGER, Examples: [2011, 2010, 2002]),
(LatestTradeData:INTEGER, Examples: [2013, 2006, 2012]),
(LatestWaterWithdrawalData:INTEGER, Examples: [2000, 2006, 2001])
]
# Table: CountryNotes
[
(Countrycode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(Seriescode:TEXT, Primary Key, Examples: [EG.EGY.PRIM.PP.KD, EG.ELC.RNEW.ZS, EG.FEC.RNEW.ZS]),
(Description:TEXT)
]
【Foreign keys】
Footnotes.Seriescode=Series.SeriesCode
Footnotes.Countrycode=Country.CountryCode
Indicators.CountryCode=Country.CountryCode
SeriesNotes.Seriescode=Series.SeriesCode
CountryNotes.Seriescode=Series.SeriesCode
CountryNotes.Countrycode=Country.CountryCode

    【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. average number refers to avg(value); passengers carried via air transport per year refers to value where IndicatorName = 'Air transport, passengers carried'; by Bulgaria refers to CountryName = 'Bulgaria'; between 1970 to 1980 refers to Year between 1970 and 1980

    【Evidence】
    average number refers to avg(value); passengers carried via air transport per year refers to value where IndicatorName = 'Air transport, passengers carried'; by Bulgaria refers to CountryName = 'Bulgaria'; between 1970 to 1980 refers to Year between 1970 and 1980

    Please read and understand the database schema carefully, and generate an executable SQL based on the user's question and evidence. 

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'