module A
  def method_a
    puts 'instance method A'
  end

  class_methods do
    def method_a
      puts 'class method A'
    end
  end
end


class B
  include A
end

class C
  extend A
end

B.method_a # "instance method A"
B.new.method_a # "class method A"
C.method_a # "instance method A"