The physical count of inventory of a retailer was lower than shown by the perpetual records. Which of the following could explain the difference?

a. Some tags for specific inventory items had mistakenly overstated the amount of those items.

b. Credit memos for several items returned by customers had not been recorded.

c. No journal entry had been made on the retailer's books for several items returned to its suppliers.

d. An item purchased "FOB destination" had not arrived at the date of the inventory count and had not been reflected in the perpetual records.