How it's working when adding virtual keyword in destructor?

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C++

Hi, Actually am trying to learn the virtual concept but the thing is how it was working while adding the virtual keyword in class destructor.

class A
{
   public:
      int a1;
      int b1;

      A()   {  cout<<"A's Constructor\n"; }

      virtual void foo()
      {
         cout<<"Base : foo\n";
      }

      //~A()  {  cout<<"A's Destructor\n";  }
      virtual ~A()  {  cout<<"A's Destructor\n";  }
};

class B : public A
{
   public:
      B()   {  cout<<"B's Constructor\n"; }

      void foo()
      {
         cout<<"Dervied : foo\n";
      }

      ~B()  {  cout<<"B's Destructor\n";  }
};

int main()
{
   A* a1 = new A();
   cout<<"------------------Another------------------\n";
   A* a2 = new B();
   cout<<"------------------Another------------------\n";
   B* b1 = new B();
   cout<<"------------------D-->a1  Another------------------\n";
   delete a1;
   cout<<"------------------D-->a2  Another------------------\n";
   delete a2;
   cout<<"------------------D-->b1  Another------------------\n";
   delete b1;
}

How, delete of a2 object calls the destructor???

What I have tried:

Trying to learn the concept of destructor seen the v-table flow control