①C(s)+H2O(g)═CO(g)+H2 (g) △H1=a kJ•mol﹣1
②CO(g)+H2O(g)═CO2(g)+H2 (g) △H2=b kJ•mol﹣1
③CO2 (g)+3H2 (g)═CH3OH(g)+H2O(g) △H3=c kJ•mol﹣1
④2CH3OH(g)═CH3OCH3 (g)+H2O(g) △H4=d kJ•mol﹣1