Some works of art that today are recognized masterpieces were considered obscene when first created. It ttherefore follows that what is considered obscene or not obscene has changed over time. Which one of the following is an assumption on which the argument depends? ['Displays of works of art that are considered obscene change the way in which obscenity is defined.', 'Public opinion does not determine the artistic value of a work of art.', 'The number of things that are considered obscene has decreased with the passage of time.', 'Not all currently recognized masterpieces that were once considered obscene are still considered obscene.']