题目内容：
设在SQL Server 2008某数据库中有表SC（Sno，Cno，Grade），其中Grade列的类型为int。若在查询成绩时，希望将成绩按"优"、"良"、"中"、"及格"和"不及格"形式显示，则下列Case函数中正确的是（ ）
A．Case GradeWhen Grade between 90 and 100 THEN Grade = '优'
When Grade between 80 and 89 THEN Grade = '良'
When Grade between 70 and 79 THEN Grade = '中'
When Grade between 60 and 69 THEN Grade = '及格'
Else Grade = '不及格'
End
B．Case Grade
When Grade between 90 and 100 THEN '优'
When Grade between 80 and 89 THEN '良'
When Grade between 70 and 79 THEN '中'
When Grade between 60 and 69 THEN '及格'
Else '不及格'
End
C．Case
When Grade between 90 and 100 THEN Grade = '优'
When Grade between 80 and 89 THEN Grade = '良'
When Grade between 70 and 79 THEN Grade = '中'
When Grade between 60 and 69 THEN Grade = '及格'
Else Grade = '不及格'
End
D．Case
When Grade between 90 and 100 THEN '优'
When Grade between 80 and 89 THEN '良'
When Grade between 70 and 79 THEN '中'
When Grade between 60 and 69 THEN '及格'
Else '不及格'
End
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