At the onset of a bout of exercise, tissue levels briefly increase due to elevated aerobic respiration.

At the onset of a bout of exercise, tissue carbon dioxide (CO2) levels briefly increase due to elevated aerobic respiration.
Aerobic respiration, once it increases drastically for a period of time, also increases the carbon dioxide of tissues so as to create an equilibrium within the body. 

D. contract and relax

What? are u trying to do a genetic test?