This following text is for questions 77 to 78.
 

    Environmentalists say that clean coal is a myth. Of course it is: Just look at West Virginia, where whole Appalachian peaks have been knocked into valleys to get at the coal underneath and streams run orange with acidic water. Or look at downtown Beijing, where the air these days is often thicker than in an airport smoking lounge. Air pollution in China, much of it from burning coal, is blamed for more than a million premature deaths a year. That's on top of the thousands who die in mining accidents, in China and elsewhere.

    These problems aren't new. In the late 17^th century, when coal from Wales and Northumberland was lighting the first fires of the industrial revolution in Britain, the English writer John Evelyn was already complaining about the "stink and darknesse" of the smoke that wreathed London. Three centuries later, in December 1952, a thick layer of coal-laden smog descended on London and lingered for a long weekend, provoking an epidemic of respiratory ailments that killed as many as 12,000 people in the ensuing months. American cities endured their own traumas. On an October weekend in 1948, in the small Pennsylvania town of Donora, spectators at a high school football game realized they could see neither players nor ball : Smog from a nearby coal-fired zinc smelter was obscuring the field. In the days that followed, 20 people died, and 6,000 people-nearly half the town-were sickened.

    Coal, to use the economists' euphemism, is fraught with "externalities" -the heavy costs it imposes on society. It's the dirtiest, most lethal energy source we have. But by most measures it's also the cheapest, and we depend on it. So the big question today isn't whether coal can ever be "clean." It can't. It's whether coal can ever be clean enough to prevent not only local disasters but also a radical change in global climate.

What is the importance of the second paragraph?