Which of the following is the most appropriate unit to describe the rate at which water flows over niagara falls?

days per billion gallons, because the independent quantity is volume of water in billion gallons and dependent quantity is time in days
billion gallons per day, because the independent quantity is volume of water in billion gallons and dependent quantity is time in days
billion gallons per day, because the independent quantity is time in days and dependent quantity is volume of water in billion gallons
days per billion gallons, because the independent quantity is time in days and dependent quantity is volume of water in billion gallons