Option C:
The rate of change is $3 per picture.
Solution:
<em>Rate of change is same as the slope of the line.</em>
Take any two points on the graph.
(0, 20) and (20, 80) are two points on the line of the graph.
Slope of two points formula:
m = 3
The rate of change is $3 per picture.
Hence option C is the correct answer.