In North America there has been an explosion of public interest in, and enjoyment of, opera over the last three decades. The evidence of this explosion is that of the 70 or so professional opera companies currently active in North America, 45 were founded over the course of the last 30 years. The reasoning above assumes which one of the following? ['The size of the average audience at performances by professional opera companies has increased over the past three decades.', 'All of the 70 professional opera companies are commercially viable operations.', 'There were fewer than 45 professional opera companies that had been active 30 years ago and that ceased operations during the last 30 years.', 'The 45 most recently founded opera companies were all established as a result of enthusiasm on the part of a potential audience.']