The equation d= -6t + 10 represents the distance d, in miles, Ralph walks from the library to his house in t hours. His sister, Joan, leaves the library right after Ralph. She rides her bicycle from the library to their house at a constant rate. The equation d = 14t represents Joan's bicycle ride where d, is the distance, in miles, and t is the time, in hours.

After how many miles will Joan meet Ralph walking to their house?
A. 2.5 miles
B. 7 miles
C. 18 miles
D. 28 miles