The price of ceramic tile at a discount tile store is represented on the graph below.

What is the meaning and value of r at Point C?

A. r represents the unit cost of ceramic tile, $0.80 for 1 ft2
B. r represents the unit cost of ceramic tile, $1.25 for 1 ft2
C. r represents the rate of increase of tile area per dollar, 0.80 ft2 per dollar
D. r represents the rate of increase of tile area per dollar, 1.25 ft2 for $1
Help please..