washington u.s. appeal court thursday grant request trump administration put litigation hold state industry group challenge obama administration pollution control rule mercury toxic emissions power plant u.s. court appeal district columbia circuit cancel oral arguments due take place may 18. u.s. environmental protection agency say want review rule rule know mat rule place years utilities already comply upgrade shut old coal-fired power plant current epa administrator scott pruitt former attorney general oklahoma one 15 state prosecutors sue previous administration block mat rule well clean power plan water unite state rule appeal court grant trump request delay mercury air pollution case