How many hydrogen ions can complex I pump across the inner mitochondrial membrane?

1. Complex I receives two electrons aboard NADH and uses their energy to move protons. 2. It can pump four hydrogen ions across the membrane from the matrix into the intermembrane space. 3. That pumping is how the hydrogen ion gradient is established and maintained between the two compartments. 4. The two compartments concerned are separated by the inner mitochondrial membrane. 5. Answering two confuses the count of pumped protons with the count of electrons delivered by NADH. 6. No figure of three or six is attached to this complex. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.4 Oxidative Phosphorylation_