John is driving around town. When he reaches the gas station, he notes that he has traveled 18 miles. He reaches home 2 hours later and notes that he has traveled 30 miles. If d represents the distance and t represents the time, in hours, John has traveled since the gas station, which of the following equations can be used to model this situation?

A. d = 18t + 48
B. d = 6t + 18
C. d = 6t - 18
D. d = 12 + 18t