Find the university from which the professor who advised most undergraduate students graduated.
university from which the professor graduated refers to graduate_from; undergraduate students refers to type = 'UG';
SELECT T1.graduate_from FROM prof AS T1 INNER JOIN RA AS T2 ON T1.prof_id = T2.prof_id INNER JOIN student AS T3 ON T2.student_id = T3.student_id WHERE T3.type = 'UG' GROUP BY T1.prof_id ORDER BY COUNT(T2.student_id) DESC LIMIT 1