The following graph shows the approximate value V(t) of subprime (normally classified as risky) mortgage debt outstanding in the United States. Subprime debt outstanding 1,400 1,200 1,000 800 S (Billions) 600 400 200 0 0 2 6 2000 2008 Year (1) (a) Use the graph to estimate, to one decimal place, the average rate of change of V(() with respect to t over the interval [0, 4] and Interpret the result. The average rate of change is , which means that the subprime mortgage debt outstanding was increasing at an average rate of around $ billion per year between 2000 and 2004. (b) Over which 2-year period(s) was the average rate of change of V() the least? (Select all that apply.) O [2, 4] O [4, 6] [6, 8]