We would over populate the eart leading to no water or no food, we would eventually go into a fallout and have nuculer war and then everything is gonna go to sh and we are going to have to repopulate
A is yours answer I think
The answer in the space provided is anaerobic respiration.
It is because this allows of having to utilize other substance than oxygen such
as nitrate or sulfate to be their electron acceptors in which is for organisms
that undergoes respiration.