假定学号的第3、4位为专业代码。要计算各专业学生选修课程号为“101”课程的平均成绩，正确的SQL语句是( )。

A) SELECT 专业 AS SUBS(学号，3，2)，平均分 AS AVG(成绩) FROM 选课
WHERE 课程号="101" GROUP BY 专业
B) SELECT SUBS(学号，3，2) AS 专业，AVG(成绩) AS 平均分 FROM 选课
WHERE 课程号="101" GROUP BY 1
C) SELECT SUBS(学号，3，2) AS 专业，AVG(成绩) AS 平均分 FROM 选课
WHERE 课程号="101" ORDER BY 专业
D) SELECT 专业 AS SUBS(学号，3，2)，平均分 AS AVG(成绩) FROM 选课
WHERE 课程号="101" ORDER BY 1