题中假定学生关系是S(S#,SN.,SEX,AGE)，课程关系是C(C#,CNAME,TEACH-ER)，学生选课关系是SC(S#,C#,GRADE)。

检索选修四门以上课程的学生总成绩(不统计不及格的课程)，并要求按总成绩降序排列出来，正确的SELECT语句是______。

A．SELECT S#,SUM(GRADE)FROM SC
WHERE GRADE＞=60
CROUP BY S#
ORDER BY 2 DESC
HAVING COUNT(*)＞=4

B．SELECT S#,SUM(GRADE)FROM SC
WHERE GRADE＞=60
GROUP BY S#
HAVING COUNT(*)＞=4
ORDER BY 2 DESC

C．SELECT S#,SUM(GRADE)FROM SC
WHERE GRADE＞=60
HAVING COUNT(*)＞=4
GROUP BY S#
ORDER BY 2 DESC

D．DELECT S#,SUM(GRADＥ)FROM SC
WHERE GRADE＞=60
ORDER BY 2 DESC
CROUP BY S#
HAVING COUNT(*)＞=4

紫外可见分光光度计根据什么原理设置？