Among students registered for the most difficult course, list the students' full names who got grade A.
difficulty refers to diff; most difficult course refers to MAX(diff); student's full names = f_name, l_name;
SELECT T1.f_name, T1.l_name 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' ORDER BY T3.diff DESC LIMIT 1