针对数据库S_T，以下创建视图的命令不正确的是（ ）。 A: create view view1asselect sno,avg(grade)from scgroup by sno B: create view view2asselect sno,sname,sdeptfrom studentwhere sdept='CS' C: create view view3asselect sno,sage+20from studentwhere ssex='女' D: create view view4asselect student.sno,sname,sc.sno,cno,grade from sc,studentwhere sc.sno=student.sno and cno='1'