When Nikki and Kara worked together last year, they painted a fence in 8 hours. The previous year, Nikki painted the fence by herself. The year before that, Kara painted it by herself, but it took 12 hours longer. How long did it take Kara to paint the fence?

a. 8 hours
b. 24 hours
c. 12 hours
d. 20 hours