单项选择题

列出SC表中所有考试不及格的学生的学号Sno（无须重复），其SQL语句为（       ）
A、SELECT DISTINCT Sno FROM SC WHERE Grade<60;
B、SELECT DISTINCT Sno FROM SC WHERE Grade IS NULL;
C、SELECT Sno FROM SC WHERE Grade<60;
D、SELECT ALL Sno FROM SC WHERE Grade<60;

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