That the application of new technology can increase the productivity of existing coal mines is demonstrated by the case of Tribnia' s coal industry. Coal output per miner in Tribnia is double what it was five years ago, even though no new mines have opened. Which of the following can be properly concluded from the statement about coal output per miner in the passage? ['Any individual Tribnian coal mine that achieved an increase in overall output in the past five years has also experienced an increase in output per miner.', 'In Tribnia the cost of producing a given quantity of coal has declined over the past five years.', "If the number of miners working in Tribnian coal mines has remained constant in the past five years, Tribnia's total coal production has doubled in that period of time.", 'If any new coal mines had opened in Tribnia in the past five years, then the increase in output per miner would have been even greater than it actually was.']