New York Times: Exelon, one of the country’s largest utilities, said Monday that it would quit the United States Chamber of Commerce because of that group’s stance on climate change. It was the latest in a string of companies to do so, perhaps a harbinger of how intense the fight over global warming legislation could become. “The carbon-based free lunch is over,” said John W. Rowe, Exelon’s chief executive. “Breakthroughs on climate change and improving our society’s energy efficiency are within …