A ramp with a constant incline connects a driveway to a front door. At a point 4 feet from the driveway, the height of the ramp is 12 inches. At a point 6 feet from the driveway, the height of the ramp is 18 inches. What is the rate of change of the ramp's incline?

A. [tex]\(\frac{1}{3}\)[/tex] inch up per foot across
B. [tex]\(\frac{1}{2}\)[/tex] inch up per foot across
C. 2 inches up per foot across
D. 3 inches up per foot across