Imagine a bathtub is being filled with water. The graph below shows the rate at which water flows from the faucet (inflow) over a 40-minute period. From minute 20 to minute 40, the faucet is left on at a steady, constant rate. Assuming the drain is only able to remove water much more slowly than the faucet's constant rate, what happens to the total amount of water in the tub (the stock) between minute 20 and minute 40?