[N_(2(g))+2O_(2(g))rarr2NO_(2)+XKJquad 2NO_((g))+O_(2(g))rarr2NO_(2(g))+YKJ],[" The enthalpy of formation of NO is "],[[" 1) "(2X-2Y)," 2) "X-Y," 3) "(Y-X)/2," 4) "(X-Y)/2]]