Use the following scenario to answer the question: A cell has an antiport protein on its apical surface. The cell is placed in a solution that contains a high concentration of Na⁺ relative to the inside of the cell, and a high concentration of Ca⁺⁺ relative to the inside of the cell. The antiporter will move 3 Na⁺ into the cell in exchange for one Ca⁺⁺ leaving the cell. This is an example of what type of transport?