The correct answer is B) aqueduct.
This is a photograph of a Roman aqueduct.
In ancient Roman times, the aqueduct was a water channel built by the Romans to convey water to the cities. The Romans needed a permanent flow of water to be used by the people of Rome in baths, latrines, basically. Aqueducts -made by rocks and bricks- were built with a downward gradient because gravity was the key for the water to move. Most aqueducts included water deposits or tanks that served to regulate the level of water.