Suppose the first hour of study today will allow you to answer 10 correct questions in tomorrow's exam. After the first hour, the marginal benefit of each additional hour of study is reduced by 2 (meaning if you study 2 hours today your total number of correct responses tomorrow will be 18 and if you study 3 hours it will be 24 and so on). If the costs of studying each additional hour are zero, how many hours you should study?

a. 6

b. 10

c. 18

d. 0

Please show your work on how you got the answer.