Which upper middle income country has the lowest value of CO2 emissions (kt)?
IncomeGroup = 'Upper middle income'; IndicatorName = 'CO2 emissions (kt); the lowest value refers to MIN(Value);
SELECT T1.CountryCode FROM Country AS T1 INNER JOIN Indicators AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IncomeGroup = 'Upper middle income' AND T2.IndicatorName = 'CO2 emissions (kt)' ORDER BY T2.Value ASC LIMIT 1