The average high temperature during the hottest month in Beijing, China, is 72°F. The average high temperature during the coldest month in Beijing is 18°F. The function t = 27 cosine (pi/6(m-6)) + 45, where m represents the number of months after January, models the temperature throughout the year in Beijing.
What is the first month in the year when the temperature reaches 55°F?
A. March
B. April
C. July
D. September