How to assign value same variable name in base class and derived class C# pin

public   class clsb
   {
       public string subjectName = "";
       public string mainsubject()
       {
          return subjectName;

       }
   }

 public  class clsA : clsb
   {
       public string subjectName = "";
       public string Allsubject()
       {
               return subjectName;
       }
   }

   private void getdata()
       {
                clsA obj = new clsA();
               obj.subjectName = "English";
               string A2 = obj.mainsubject();
               MessageBox.Show(A2);
  }