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 gradeWhen 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