When a plant loads sucrose into a phloem sieve tube element at a source, which row shows the correct use of proton pumps ($\text{H}^+$ pumps) and co-transporters?
- Aproton pump pumps protons into a companion cell raising its pH; co-transporter uses the high concentration of protons in the companion cell to move sucrose out of the cell
- Bproton pump pumps protons into a mesophyll cell raising its pH; co-transporter uses the high concentration of protons in a mesophyll cell to drive sucrose into the cell
- Cproton pump pumps protons out of a companion cell into its cell wall; co-transporter uses the high concentration of protons outside a companion cell to drive sucrose into the cell
- Dproton pump pumps protons out of a mesophyll cell into its cell wall; co-transporter uses the low concentration of protons in a mesophyll cell to move sucrose out of the cell