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

: Day 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

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