现要查找缺少成绩(Grade)的学生学号(Snum)，相应的SQL语句是

A.SELECTSnumFROMSCWHEREGrade=0

B.SELECTSnumFROMSCWHEREGrade<=0

C.SELECTSnumFROMSCWHEREGrade=NULL

D.SELECTSnumFROMSCWHEREGradeISNULL

正确答案：SELECTSnumFROMSCWHEREGradeISNULL