Consider the following statements: I. The standard cost per unit of materials is used to calculate a materials price variance. II. The standard cost per unit of materials is used to calculate a materials quantity variance. III. The standard cost per unit of materials cannot be determined until the end of the period. Which of the above statements is (are) true?

a. I, II, and III.
b. II only.
c. I only.
d. I and II.
e. III only.