Which of the Following Must Be True in Order for Materials

Which of the following must be true in order for materials to be classified as direct materials?

A) They must be classified as both prime costs and conversion costs.
B) They must be introduced into the process in both work in process inventories and finished goods inventories.
C) They must be an integral part of the finished product, but can be an insignificant portion of the total product cost.
D) They must be an integral part of the finished product and be a significant portion of the total product cost.