With oxygen present, the Krebs cycle and the electron transport chain A. provide organisms an alternative to glycolysis. B. produce most of the ATP needed for life. C. break down glucose to produce carbon dioxide, water, and ATP. D. All of the above

With oxygen present, the Krebs cycle and the electron transport chain C. break down glucose to produce carbon dioxide, water and ATP.

Explanation:

In glucose metabolism the first stage is glycolysis. This includes the breakdown of glucose to produce ATP. The end product of this is pyruvate. The Krebs cycle and the electron transport chain occur inside the mithocondria. The Krebs cycle  will oxidate the pyruvate. The NADH and FADH2 produced by the krebs cycle will be used in the electron transport chain. The result of all these will to produce carbon dioxide, water, and ATP

geologic time scale (for gradpoint) =)