Used in medium

access_module_exclude.rb

require_relative '01.define_module'
extend Test

class C_test
  p self.ancestors  # => [C_test, Object, Kernel, BasicObject]
  p self.singleton_class.ancestors
  # => # => <Class:Klass>, Test, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]
end

C_test.new.hello   # => undefined method `hello' for #<C_test:0x0000555e680067e0> (NoMethodError)
C_test.hello   # => undefined method `hello' for C_test:Class (NoMethodError)
hello  # => I'm an instance method.