18. Bart's indexed annuity uses a long-term point-to-point interest crediting strategy over a three-year period. If the index closing level is 1000 at the beginning of the index term period, and the closing levels at the end of the following three years are 900,1050 , and 1100, what is the percentage change for the three-year period? a. 5 percent b. 10 percent c. 15 percent d. 21.5 percent