In (a),the price is shown rising over time.Units of a resource in the ground must earn a return commensurate with that on other assets.Therefore,in a competitive market,price less marginal production cost will rise at the rate of interest. Part (b)shows the movement up the demand curve as price rises. In (a), the price is shown rising over time. Units of a resource in the ground must earn a return commensurate with that on other assets. Therefore, in a competitive market, price less marginal production cost will rise at the rate of interest. Part (b) shows the movement up the demand curve as price rises