Question

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

Answer 1

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

Hope this helps!!

Answer 2

Here "rate" refers to how much water in how much time, so eliminate A qnd D.  Which sounds correct, B or C?  The correct answer corresponds to 
(quantity of water) / (period of time).