The abstract classes

Question

 The abstract classes are base classes.The abstract classes are always having child classes.The abstract classes doesn't having instance variables.The abstract classes having reference variables.

Example on Abstract Class:

abstract class Dept1
{
protected int sal;
public void eSal()
{
sal=5000;
}
public abstract void tSal();
}
class Dept2:Dept1
{
int bonus;
public override void tSal()
{
bonus=500;
int ts=sal+bonus;
MessageBox.Show(ts.toString());
}
}
private void button1_click()
{
dept2 obj=new Dept2();
obj.eSal();
obj.tSal();
}