Assertion (A): Dalton's atomic theory, which treats a chemical reaction as a reorganisation of atoms, could explain Gay Lussac's finding that two volumes of hydrogen and one volume of oxygen give two volumes of water vapour at the same temperature and pressure. Reason (R): Avogadro proposed that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules, and he took the molecules of hydrogen and oxygen to be made of two atoms each.