In the $ETS$ (Electron Transport System),how many molecules of each cytochrome are required for the transfer of $2$ $e^-$?

The chemiosmotic theory of oxidative phosphorylation suggests that adenosine triphosphate $(ATP)$ is produced because:

Given below are two statements.
Statement-$I$: During aerobic respiration,$ETS$ regenerates co-enzymes such as $NAD^+$ and $FAD^+$ from their reduced forms $NADH + H^+$ and $FADH_2$.
Statement-$II$: During aerobic respiration,$ETS$ generates a major amount of energy in the form of $ATP$ molecules.
In the light of the above statements,choose the correct answer from the options given below.

Identify the site of oxidative phosphorylation.

Which of the following components of $ETS$ acts as a mobile electron carrier?

Oxygen is an essential requirement for aerobic respiration, but it enters the respiratory process at the end. Discuss.