The current pattern of human consumption of resources, in which we rely on nonrenewable resources, for example metal ore, must eventually change. Since there is only so much metal ore available, ultimately we must either do without or turn to renewable resources to take its place. Which one of the following is an assumption required by the argument? ['We cannot indefinitely replace exhausted nonrenewable resources with other nonrenewable resources.', 'Consumption of nonrenewable resources will not continue to increase in the near future.', 'There are renewable resource replacements for all of the nonrenewable resources currently being consumed.', 'Ultimately we cannot do without nonrenewable resources.']