Which of the following are examples of combination reaction?
(i) CaO+O2→2CaCO3
(ii) CaCO3→CaO+CO2
(iii) MgO+CO2→MgCO3
(iv) Fe+S→FeS
(v) 2AgCl+sunlight→2Ag+Cl2
(vi) CH4+2O2→CO2+2H2O
(vii) Na2O+H2O→2NaOH