what happens at the end of the chain in figure 9.3?; a) 2 electrons combine with a proton and a molecule of nad���.; b) 2 electrons combine with a molecule of oxygen and two hydrogen atoms.; c) 4 electrons combine with a molecule of oxygen and 4 protons.; d) 4 electrons combine with four hydrogen and two oxygen atoms.; e) 1 electron combines with a molecule of oxygen and a hydrogen atom.