The following six months of data were collected on electricity cost and the number of machine hours in a factory.

Month	Electricity cost	Machine hours
June	$25,160	4,500
July	26,170	4,810
August	27,250	5,120
September	26,680	5,010
October	27,950	5,430
November	27,500	5,190

Refer to Figure 3-13. An independent variable value used in calculating the cost line using the high-low method is:

a.$27,950

b.4,000

c.5,430

d.$25,160