听力原文：M: As a means of international settlement, what is the main role of a credit card？
W: Its main role is to transfer the payment obligation from payer to bank. It makes the payment safer for the exporter with the bank' s credit and guarantee, and simultaneously makes the supply of goods safer for the importer.
Q: What is the main role of a letter of credit？
(19)

A. It is to transfer the contract from bank to customer.
B. It is to transfer the payment obligation from buyer to seller through banks.
C. It is to transfer the confirmation from the issuing bank to confirming bank.
D. It is to transfer the documents from advising bank to issuing bank.