Which of the following equation is correct in respect of raw materials available for use? Select one: a. Opening stock of raw materials + purchase of raw materials + carriage inwards returns outwards closing stock of raw materials b. Opening stock of inventories + purchase of raw materials + carriage inwards returns outwards closing stock of inventories c. Opening stock of raw materials + purchase of raw materials + work-in-progress + carriage inwards returns outwards closing stock of raw materials d. Opening stock of raw materials + purchase of raw materials + carriage inwards returns outwards