Discuss the efficient extraction an exhaustible resource by explaining the behavior of extraction, price and user cost over time and the conditions that define the date at which extraction ceases. Begin by assuming that demand and extraction cost are.stable over time, that there is no substitute for the resource, and that there are no externalities associated with the extraction or use of the resource. Next, explain the differences that arise if an abundant, renewable substitute is avallable at a constant marginal cost that is less than the choke price of the exhaustible resource in the absence of a substitute