static void Main(string[] args)
{
Person person = new Person();
Person student = new Student();
Person teacher = new Teacher(30);
person.SayHello();
student.SetAge(21);
student.SayHello();
student.ShowAge();
teacher.SayHello();
teacher.Explain();
}
static void Main(string[] args)
{
Person p = new Student();
p.SayHello();
Student s1 = new Student();
s1.SetAge(21);
s1.SayHello();
s1.ShowAge();
Teacher t = new Teacher();
t.SetAge(30);
t.SayHello();
t.Explain();
}