Bacteriorhodopsin pumps protons across the bacterial plasma membrane from the cytosolic side to the extracellular side of the membrane in a series of steps. Choose the step that correctly describe a step used to pump protons across the membrane. A. A proton is transferred from excited retinal to the Asp96 side chain. B. The chromaphore retinal absorbs a photon of light, causing a double bond in retinal to isomerize from trans to cis configuration. C. A proton from Asp96 on the cytosolic side of the protein is used to regenerate retinal. D. A proton from the cytosol side is transferred along multiple water molecules to Asp85.