Fact 1: Electron transport is the final stage of aerobic respiration. Fact 2: Aerobic respiration occurs only in the presence of oxygen. Given the two facts above, electron transport requires which of the following?

Choose from:
1). electron microscope
2). sodium
3). Energy.
4). Oxygen
5). fission
6). warmth
7). flagellum
8). Energy
Answer:
4).