Many Vitamins Are Reported in IU's or International Units Rather

Many vitamins are reported in IU's or International Units rather than masses. Which of the following statements best explains the reason for this?

A) It represents all potential forms and equivalent compounds of the vitamin as well.
B) It represents an easy conversion in countries that utilize the metric system
C) It allows for the dosage of the vitamin to be expressed in tablets rather than mass.
D) All of the above
E) None of the above