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A. When the pH value of sewage is kept between 5.0 and 6.0 and Fe(OH)3 precipitates are generated by electrolysis, the electrolyte added to enhance conductivity must be water-soluble neutral salt. Adding sulfuric acid can't produce Fe(OH)3 precipitate, so A is wrong;

B.Fe is the anode in the electrolytic cell, and Fe-2e-=Fe2+ appears, so B is wrong;

C. The battery uses molten carbonate as electrolyte, the negative reaction is CH4+4Co32-8e-= 5co2+2h2o, the positive reaction is O2+2CO2+4e-=2CO32-, and the only substance that can be recycled is carbon dioxide, so C is correct;

D. The electrode reaction of the cathode is 2h+2e-= H2 ↑, and the cathode produces 44.8L (standard state), that is, 2mol of hydrogen, so the amount of substances that transfer electrons is 4mol. According to the negative reaction of the battery, CH4+4CO32-8e-= 5co2+2h2o, and the second device has 1.6 gCH4.

So choose C.