In the electron transport system

a. nadh and fadh2 both donate electrons at the same location.
b. nadh donates electrons "upstream" of where fadh2 donates electrons.
c. nad donates electrons at the head of the chain.
d. nadp donates electrons in the middle of the chain.

A. NADH and FADH2 both donate electrons at the same location.

Explanation:

In the respiratory chain, four large protein complexes inserted into the mitochondrial inner membrane transport NADH and FADH₂ electrons (formed in glycolysis and the Krebs cycle) to oxygen gas, reducing them to NAD⁺ and FAD, respectively.

These electrons have great affinity for oxygen gas and, when combined with it, reduce it to water molecules at the end of the reaction.

Oxygen gas effectively participates in cellular respiration at this stage, so its absence would imply interruption of the process.

NADH and FADH₂ electrons, when attracted to oxygen, travel a path through protein complexes, releasing energy in this process.

The energy released by the NADH and FADH₂ electrons in the respiratory chain in theory yields 34 ATP, however, under normal conditions an average of 26 ATP molecules is formed.

If we consider that these 26 molecules are added to the two ATP formed in glycolysis and two ATP formed in the Krebs cycle, it can be said that cellular respiration reaches a maximum yield of 30 ATP per glucose molecule, although theoretically this number was 38 ATP per glucose molecule.

doctors i believe if i'm wrong sorry