在学生选课数据库S_T中，有:
学生表student(sno,sname,ssex,sage,sdept)，
课程表course(cno,cname,cpno,ccredit)，
选课表sc(sno,cno,grade)。
针对数据库S_T，以下创建视图的命令不正确的是（      ）。
A、create view view1
as
select sno,avg(grade)
from sc
group by sno
B、create view view2
as
select sno,sname,sdept
from student
where sdept='CS'
C、create view view3
as
select sno,sage+20
from student
where ssex='女'
D、create view view4
as
select student.sno,sname,sc.sno,cno,grade 
from sc,student
where sc.sno=student.sno and cno='1'