Which of the following is true concerning the materials price variance? a. It is the difference between the actual and standard unit price of an input multiplied by the number of inputs used. b. It is the difference between the actual and standard unit price of an output multiplied by the number of inputs purchased. c. It is the difference between the actual and standard unit price of an input multiplied by the number of inputs purchased. d. It is the difference between the actual and standard unit price of an output multiplied by the number of inputs used. e. None of these choices are correct.