ambiguity in inheritance c++

       #include<iostream.h>
       #include<conio.h>

       class ClassA
       {
              public:
              int a;
       };

       class ClassB : public ClassA
       {
              public:
              int b;
       };
       class ClassC : public ClassA
       {
              public:
              int c;
       };

       class ClassD : public ClassB, public ClassC
       {
              public:
              int d;
       };

       void main()
       {

			  ClassD obj;

			  //obj.a = 10;                   //Statement 1, Error occur
			  //obj.a = 100;                 //Statement 2, Error occur

			  obj.ClassB::a = 10;        //Statement 3
			  obj.ClassC::a = 100;      //Statement 4

			  obj.b = 20;
			  obj.c = 30;
			  obj.d = 40;

			  cout<< "\n A from ClassB  : "<< obj.ClassB::a;
			  cout<< "\n A from ClassC  : "<< obj.ClassC::a;

			  cout<< "\n B : "<< obj.b;
			  cout<< "\n C : "<< obj.c;
			  cout<< "\n D : "<< obj.d;

	   }

   Output :

              A from ClassB  : 10
              A from ClassC  : 100
              B : 20
              C : 30
              D : 40