Art historian: Successful forgeries tend to be those most recently painted. While such a phenomenon may sound counterintuitive, a forger is able to exploit current modes of aesthetics to create paintings that appeal to the eye of his or her contemporaries. This very quality, however, is what makes those paintings seem amateurish to subsequent generations. As a result, a majority of forgeries are identified as such roughly twenty-five years after their creation. Which of the following is an assumption upon which the argument rests? ['A piece of art determined to be a forgery does not, after a period of twenty-five years, become valued for its own intrinsic merits.', 'A generation consists of exactly twenty-five years.', 'Those who expose the majority of paintings as forgeries are guided by aesthetic considerations.', 'What is deemed aesthetically pleasing does not change in the course of twenty-five years.']