nventory carrying costs $\_\_\_\_\_.$

a$.$ include the costs of generating memos, fax transmissions, etc. associated with placing an order with a supplier

b$.$ generally increase in proportion to the average amount of inventory held

c$.$ are equal to the sum of total inventory costs and total ordering costs

d$.$ are at maximum when a firm's order is equal to optimal quantity

e$.$ are fixed regardless of the average size of the inventory