Economist: Paying extra for fair-trade coffee-coffee labeled with the Fairtrade logo-is intended to help poor farmers, because they receive a higher price for the fair-trade coffee they grow. But this practice may hurt more farmers in developing nations than it helps. By raising average prices for coffee, it encourages more coffee to be produced than consumers want to buy. This lowers prices for non-fair-trade coffee and thus lowers profits for non-fair-trade coffee farmers.To evaluate the strength of the economist`s argument, it would be most helpful to know which of the following?
【选项】What proportion of coffee farmers in developing nations produce fair-trade coffee