I'm not sure how this answer is correct. I used the Formula (CPI2011-CPI2008)/CPI2008. Then divide the number by 4 years to get the average. I get 2.86%

1) Suppose that the CPI was 105 on Jan 1, 2008 and 117 in Jan 1, 2011. What was the average annual inflation over the intervening period? A) 4.00% B) 3.97% C) 3.87% D) 3.77% E) 3.67% Answer: E