单选题 The era of cheap and convenient sources of energy is coming to an end. A transition to more expensive but less polluting sources must now be managed. Energy contributes positively to human's well-being by providing such consumer services as heating and lighting. But the costs of energy—including not only the money and other resources devoted to exploiting it but also environmental impacts—diminish the well-being. For most of the past 100 years, the problems of excessive energy costs have seemed less threatening than the problems of insufficient supply. However, it became possible to think that expanding some forms of energy supply could create costs exceeding the benefits as energy was getting costlier in all respects. There are a variety of other energy resources that are more abundant than oil and gas. But none of them can deliver large quantities of electricity at costs comparable to those of the cheap coal-fired plants of the 1960's.