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b.fermentation ....... because acetyl groups produced from breaking down fats or proteins can also combine with co-enzyme A tojoin krebs cycle so glycolosis and formation of pyruvic acid arent essential for cellular respiration .while in case of fermentation, glycolosis is essential to produce two molecules of pyruvic acid which are reduced in the abscence or lack of oxygen either into lactic acid in muscles and bacteria through acidic fermentation or into ethyl alcohol and carbon dioxide in yeast and plant cells through alcoholic fermentation