Which one is not correct for transferable credit and back-to-back credit? ( )

A、Two credits are involved in transferable credit while one letter of credit is involved in back-to-back credit.
B、One credit is involved in transferable credit while two letters of credit are involved in back-to-back credit.
C、They are both used by middlemen.

D、Under the back to back L/C, the applicant is required to provide the original L/C for mortgage while it is not for transferable credit.