They bind onto fad to form the fadh two. And these two electrons left over here, one here and one here, create a double bond to form this fumarate molecule. And the enzyme that catalyzes this step of the citric acid cycle is known as Succinate dehydrogenase. And actually, Succinate dehydrogenase, the enzyme that catalyzes this step of the citric acid cycle, is found within complex two. So complex two is actually involved in the citric acid cycle in forming the fadh two molecule and extracting those electrons from Succinate to form fumerate. Now, complex two, for that reason, is also known as Succinate reductase, because we essentially abstract those electrons from Succinate, we oxidize it into fumerate, and we reduce the fad into fadh two.