The number of hours of daylight each day for a particular city can be modeled using the sinusoidal function s(m)=5sin ( 2π /12 m- π /2 )+13 , where 8 is the number of hours of sunlight per day, and m represents the month of the year, with m=1 corresponding to January, m=2 corresponding to February, etc.

What is the number of hours of daylight in the longest day of the year?

A. 8 hours
B. 13 hours
C. 18 hours
D. 24 hours