What is the difficulty of the course in which a student with level of intellengence of 5 got an A grade?
difficulty of the course refers to diff;
SELECT T3.diff FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T2.grade = 'A' AND T1.intelligence = 5