题目内容：
在关系SC中，查询学习课程号为'DB'，且成绩在60至90之间的学生学号的正确的SQL命令是（ ）。
A．SELECT SNO FROM SC WHERE CNO='DB'AND GRADE BETWEEN 60 AND 90 B．SELECT SNO FROM SC WHERE CN0='DB' 0R GRADE BETWEEN 60 AND 90
C．SELECT SNO FROM SC WHERE CNO='DB'AND GRADE>=60 AND 90
D．SELECT SNO FROM SC WHERE CNO='DB'AND GRADE<=60 AND GRADE>=90
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答案解析：