The Administrator, in consultation with the Secretary of Energy, must conduct a study that addresses a number of specific factors regarding, among other  the need for further reductions of these pollutants from affected EGUs and other sources to attain or maintain the national ambient air quality standards; whether adjusting any of the limitations would have benefits that would justify the costs of such adjustment; the relative marginal cost effectiveness of the reductions from affected EGUs; and the feasibility of attaining the limitations and any alternative limitations.