Data regarding Rock Corp.’s available-for-sale securities follows: Cost Market Value December 31, year 8 $ 80,000 $ 65,000 December 31, year 9 $ 80,000 $ 90,000 Differences between cost and market values are considered temporary. Rock does not elect the fair value option of accounting for available-for-sale securities. By what amount should Rock increase (credit) its year 9 other comprehensive income?