Question

oxidation-reduction reactions
never occur at the same time.
remove electrons from both molecules involved.
remove protons from one molecule and join them to another molecule.
occur simultaneously.
remove oxygen from the cell.

Explanation:

Oxidation - reduction (redox) reactions involve the transfer of electrons. In a redox reaction, oxidation (loss of electrons) and reduction (gain of electrons) must occur simultaneously because one substance loses electrons (is oxidized) and another substance gains those electrons (is reduced). So the statement "never occur at the same time" is wrong. Redox reactions don't remove electrons from both molecules (one loses, one gains), so that option is wrong. They don't involve proton transfer (that's more related to acid - base reactions), so that option is wrong. And they don't remove oxygen from the cell (the name redox can involve oxygen, but the key is electron transfer), so that option is wrong. The correct statement is that they occur simultaneously.

Answer:

occur simultaneously.