The Answer Is A. Hope This Helps ;P

<h2>Fixation</h2>
Nitrogen in its gaseous form (N2) can’t be used by most living things. It has to be converted or ‘fixed’ to a more usable form through a process called fixation. There are three ways nitrogen can be fixed to be useful for living things:
<h3>Biologically: </h3>
Nitrogen gas (N2) diffuses into the soil from the atmosphere, and species of bacteria convert this nitrogen to ammonium ions (NH4+), which can be used by plants. Legumes (such as clover and lupins) are often grown by farmers because they have nodules on their roots that contain nitrogen-fixing bacteria.
<h3>Through lightning: </h3>
Lightning converts atmospheric nitrogen into ammonia and nitrate (NO3) that enter soil with rainfall.

The second phage of cellular respiration is transition stage.
Process take place in transition stage:
The transition stage take place in mitochondria. The pyruvate is combined with NAD+ to form NADH and acetyl co-enzyme molecules.
After transition stage, Krebs cycle starts.
not sure, but its part of your heamoglobin which is part of a component of your blood
It's controlled by the Hypothalamus.