已知表:SC(S#,C#,CRADE),其中S#、C#、CRADE分别代表学号、选修课号、成绩。则统计选修了课程的学生的总数的SQL语句应为: A: SELECT COUNT(DISTINCT S B: ) FROM SC C: SELECT COUNT(DISTINCT *) FROM SC D: SELECT COUNT(S E: ) FROM SC F: SELECT COUNT(DISTINCT C G: ) FROM SC