The essence of biochemistry

I'm now dwelling in metabolism, the reason why biochemistry exist in the first place. The lecturer is such a funny guy, he always make seemingly mundane reactions fun to learn.

Multiple choice questions are part and parcel of academic life, from the time I was a kid till university. The answer of MCQ is revealed, all you need to do is just get rid of distractions to be deemed righteous. But when things don't seem to work out (at times), there's still tricks to hit it right, for near silly reasons:

1. The answer sounded good (and grammatically correct! :)
2. Look at the rest of the questions. Do one option seem to dominate your answer choice? Just choose that option, by chance you'll get it right :P (eg. if you notice many questions have A as their answer choice, pick A if you dunno the ans!)
3. The choice with the longest sentence as answer (and it sounds wise!)

In everyday life, we use money to fulfil our needs and desires, nobody provide favour for nothing. The same goes for cells in our body, life processes are driven by energy. Some molecules serve as energy "currencies" in cells:

This is called ATP. Like a one dollar note of energy

ATP is called the "universal currency of energy". Whether you're a bacteria, worm, plant, animal or human, cells in your body uses ATP as "money" to pay for life processes.

This is a 50 dollar note of metabolic energy. It's called NAD.

Central to your use of oxygen to gain the most out of your food, is the grand roundabout of metabolism:

The Krebs cycle.

You ask me, where is the oxygen in the "roundabout"?