[单项选择](37)—(38)题基于学生一课程数据库中的三个基本表: 学生信息表:s(sno,sname,sex,age,dept)主码为sno 课程信息表:c(cno,cname,teacher)主码为cno 学生选课信息表:sc(sno,cno,grade)主码为(sno,cno) “从学生选课信息表中找出无成绩的元组”的SQL语句是 A. SELECT*FROM sc WHERE grade=NULL B. SELECT*FROM sc WHERE grade IS’’ C. SELECT*FROM sc WHERE grade IS NULL D. SELECT*FROM sc WHERE grade=’’