A.
select sname,sage,grade from student s join sc on s.sno=sc.sno
B.
select sname,sage,grade from student where sage<20
C.
select sname,sage,grade from student s join sc on s.sno=sc.sno where sage<20
D.
select sname,sage,grade from student s join sc on student.sno=sc.sno where sage<20