Andy made a spirometer for a physics experiment. The spirometer is designed to measure the volume of air in a person's lungs. Andy found that when he breathes normally, he inhales and exhales 0.5 of a liter of air per breath. He also found that when breathing normally, after exhaling, he still has 2 liters of air in his lungs.

Andy recorded 15 breaths in one minute, starting at the beginning of an exhale.

If Andy inhaled and exhaled the exact same amount of air in each breath, which of the following functions best models the amount of air in Andy's lungs t seconds after he started recording?