Among the black students in 2011, list the institution site and name of those who has 20 t0 30 degree-seeking students in the cohort.
black refers to race = 'B'; in 2011 refers to year = '2011'; institution name refers to chronname; 20 to 30 degree-seeking students in the cohort refers to grad_cohort BWEEN 20 AND 30;
SELECT DISTINCT T1.site, T1.chronname FROM institution_details AS T1 INNER JOIN institution_grads AS T2 ON T2.unitid = T1.unitid WHERE T2.year = 2011 AND T2.race = 'B' AND T2.grad_cohort BETWEEN 20 AND 30