class_variables.rb

class A
  def test
    puts "A::test"
  end 
end

class B
  def test
    puts "B::test"
    C.a.test
  end 
end

class C
  def self.test
    @@a = A.new
    @@b = B.new

    @@b.test
  end 

  def self.a
    @@a 
  end 
end

C::test