在关系SC中，查询学习课程号为’OB’，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 FROMSC WHERE CNO＝’DB’ AND GRADE BETWEEN 60 AND 90
B．SELECT SNO FROM SC WHERE CNO＝’DB’ OR GRADE BETWEEN 60 AND 90
C．SELECT SNO FROM SC WHERE CNO＝’DB’ AND GRADE＞=60 AND 90
D．SELECTSNO FROM SCWHERECNO＝’DB’ AND GRADE＜＝60 AND GRADE＞＝90