The correct answer is "change the value of n to $18"$ please explain fully how they got this answer, it makes no sense to me$.$ Let's say you want to calculate the current market price of a similar bond that was issued six years ago. The maturity was $15$ years. How would you change your method of calculating the current market price of the bond?

a$)$ don't do anything differently

b$)$ change the value of $"$n$"$ to $9$

c$)$ change the value of $"$n$"$ to $12$

d$)$ change the value of $"$n$"$ to $18$