Journalize the entries for the following transactions, for the perpetual inventory system:
a. Sold merchandise for cash, $18,500. The cost of the goods sold was $11,000.
b. Sold merchandise on account, $12,000. The cost of the goods sold was $7,200.
c. Sold merchandise to customers who used MasterCard and VISA, $115,200. The cost of the goods sold was $70,000.
d. Received an invoice from National Credit Co. for $5,600, representing a service fee paid for processing MasterCard and VISA.