37.第(37．-(38．题基于学生-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（本题分值：1分） (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 =' '