The following information is available for electricity costs for the first six months of the year. Month Production Volume Electricity Cost January 2,800 $2,100 February 5,600 3,600 March 6,400 3,800 April 3,500 2,600 May 2,400 1,600 June 4,200 2,700 Using the high-low method, estimated variable cost per unit of production would be: Select one: a. $0.59 b. $0.55 c. $0.51 d. $0.42