I'm looking for a way of making a method "personal" - note NOT PRIVATE to a class

here is an example - by "personal" I mean the behaviour of method "foo"

class A
  def foo
     "foo"
  end
end

class B < A
  def foo
      "bar"
  end
end

class C < B
end

a=A.new; b=B.new;c=C.new

I'm looking for a way of producing the following behaviour

a.foo #=> "foo"

b.foo #=> "bar"

c.foo #=> "foo" (ultimate base class method called)

Any ideas?

Thanks

Steve