单项选择题
有学生和班级两个关系，当班级中有学生时，不得删除该班级。实现这一完整性约束使用的SQL子句是（）
A.ON DELETE CASCADE
B.ON DELETE ALL
C.ON DELETE RESTRICT
D.ON DELETE SET NULL
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单项选择题
A.ON DELETE CASCADE
B.ON DELETE ALL
C.ON DELETE RESTRICT
D.ON DELETE SET NULL
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