Sean began jogging to live a healthier lifestyle. On his first run, he ran one-half mile. He increased his workouts by adding two miles a month to his run. He wrote the equation f(x)= 0.5 + 2x to model his progress. The variable x represents the number of?
A) miles he runs.
B) months he runs.
C) miles he ran the first day.
D) calories he burns.