Which university had the most students in 2011? Show its name.
in 2011 refers to year 2011; the most students refers to MAX(num_students); which university refers to university_name;
SELECT T2.university_name FROM university_year AS T1 INNER JOIN university AS T2 ON T1.university_id = T2.id WHERE T1.year = 2011 ORDER BY T1.num_students DESC LIMIT 1