Beginning in the middle of summer, the average temperature for a certain year in Phoenix could approximately be modeled by the function f(x)=26cos(0.0055πx)+89, where x represents the number of days since the middle of the summer, and f(x) represents the average temperature in degrees Fahrenheit. What was the lowest average temperature in Phoenix that year (approximately)?

a) 63∘F

b) 55∘F

c) 26∘F

d) 89∘F