The graph represents the function where electricity usage in kilowatts per hour of a clock radio varies directly with the number of days it is plugged into the wall current. A coordinate grid showing Clock Radio Electricity Usage, with Days Connected to Current on the x-axis and Electricity Usage in kilowatt hours on the y-axis with a line starting at (0, 0) and passing through (2, 0.5) and (6, 1.5).
Which is a reasonable estimate of the constant of variation?

0.25 kWh per day
0.50 kWh per day
2.40 kWh per day
4.00 kWh per day