Balance the following equations.
a. _CH4 + _O2 → _CO2 + _H2O
b. _Zn + _HCl → _ZnCl2 + _H2
c. _H2SO4 + _HI → _H2S + _I2 + _H2O
d. _NaCl + _SO2 + _H2O + _O2 → _Na2SO4 + _HCl