在“学生—选课—课程”数据库中的两个关系如下：
s(sno,sname,sex,age)
sc(sno,cno,grade)
则关系代数表达式π_{s.sname,grade}(
)等价的SQL命令是SELECT s.sname,sc.grade FROM s,sc WHERE______AND s.age＞20

A．SELECT sno FROM sc WHERE grade=NULL
B．SELECT sno FROM sc WHERE grade IS
C．SELECT sno FROM sc WHERE grade IS NULL
D．SELECT sno FROM sc WHERE grade=0