Avery leaves her house at 6:00 a.m. to go for a 9-mile run and returns to her house at 6:30 a.m. Describe the distance d (in miles) Avery runs as a linear function of her run time t (in minutes).
a) d = 0.3t
b) d = 9t
c) d = 18t
d) d = 18 + 0.3t