Hello there!
I just took the quiz, and the answer is B. The water will continue to decrease.
WHY:
First, let's look at the definition of a "decade". A decade is the equivalence to 10 years. Since the chart ends in 2010, the next decade will end in 2020. Since it is still decreasing - not that fast, not that slow - it will not completely deplete.
Hope this helped!
-Sam