Which of the two courses, "Advanced Operating System" or "Intro to BlockChain', did most of the students receive an A in?
A refers to an excellent grade in which grade = 'A';
SELECT T2.name FROM registration AS T1 INNER JOIN course AS T2 ON T1.course_id = T2.course_id WHERE T1.grade = 'A' AND T2.name IN ('Advanced Operating System', 'Intro to BlockChain') GROUP BY T2.name ORDER BY COUNT(T1.student_id) DESC LIMIT 1