Among the low income countries, which country has the lowest fertility rate?
fertility rate refers to IndicatorName = 'Adolescent fertility rate (births per 1,000 women ages 15-19)'; lowest refers to MIN(Value); IncomeGroup = 'Low income';
SELECT T2.CountryName FROM Country AS T1 INNER JOIN Indicators AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IncomeGroup = 'Low income' AND T2.IndicatorName = 'Adolescent fertility rate (births per 1,000 women ages 15-19)' ORDER BY T2.Value LIMIT 1