Can somoene explain this outcome

class myClass
 {
     public int x;
    public myClass()
     {

     }
 }


 class MainClass
 {
     public static void Main()
     {
         myClass referenceA = new myClass();
         myClass referenceB = new myClass();

         referenceA.x = 10;
         referenceB.x = 20;
         Console.WriteLine("a.x {0}, b.x {1}",referenceA.x,referenceB.x);

         referenceA = referenceB;
         referenceB.x = 30;
         Console.WriteLine("a.x {0}, b.x {1}", referenceA.x ,referenceB.x);
         Console.ReadLine();
     }
 }