Associated Press: Heat-trapping pollution from U.S. power plants hit a 27-year low in April, the Department of Energy announced Wednesday. A big factor was the long-term shift from coal to cleaner and cheaper natural gas, said Energy Department economist Allen McFarland. Outside experts also credit more renewable fuel use and energy efficiency. Carbon dioxide - from the burning of coal, oil and gas - is the chief greenhouse gas responsible for man-made global warming. "While good news for the environment, we certainly...