What are the limitations of using measures of physical scarcity such as the "exponential exhaustion index" that was developed by the Meadows in the book Limits to Growth published in 1972? What is the "Hotelling efficiency rule" and how does it relate to the "socially optimal" level of depletion of an exhaustible resource? Do you think that markets can be relied upon to allocate exhaustible resources over time to maximize soxiety's welfare? If not, what alternative mechanisms should be used?