Tanner Trucking Company is exploring different prediction models that can be used to forecast indirect labor costs. One independent variable under consideration is machine hours. Following are matching observations on indirect labor costs and machine hours for the past six months: Month Machine hours Indirect labor costs 1 300 $20,000 2 400 $24,000 3 240 $17,000 4 370 $22,000 5 200 $13,000 6 225 $14,000 In a high-low model, which months' observations would be used to compute the model's parameters? Select one: a. 4 and 5 b. 2 and 6 c. 1 and 6 d. 2 and 5