A:fetched
B:watched
C:followed
D:collected
①NH2+CO2+X=NH4HCO3
②NH4HCO3+NaCl=NH4Cl+NaHCO3↓
③2NaHCO3 Δ__ Na2CO3+H2O+CO2↑
请回答下列问题:
造成酸雨的主要物质是( )