The cost of a long-distance phone call is a function of the length of the call in minutes.

If the costs of the four calls shown in the table are graphed with minutes on the horizontal axis and cost on the vertical axis, what does the slope of the graph represent?
Long-Distance Calls Call in minutes Cost
5 $0.65
10 $1.30
15 $1.95
20 $2.60
A rate of $0.13 per minute
The total cost per call
An average time of 5 minutes between calls
An average of 12.5 minutes per cal