短文理解
听力原文： A standby credit letter (SCL) is a contingent obligation of the letter's issuer. The issuing bank agrees to guarantee the credit of its customer or to guarantee the fulfillment of a contract made by its customer with a third party. The issuing bank earns a fee from providing the service of SCL. In general, the fee is relatively low and usually around 0.5 percent to 1 percent of the amount of credit involved due to the following reasons. On the one hand, the issuing bank may have previously analyzed the financial condition of its standby credit customer, and the probability is low that the issuer of the credit guarantee will ever be called upon to pay. On the other hand, the issuing banks carry zero reserve requirements and avoid the booking of additional assets, which would also require each bank to acquire more capital.
21. Is SCL same as revocable letter of credit according to the passage？
22.What will the issuing bank do for a standby credit letter？
23.Why is the service fee of SCL relatively low？
(21)

A. almost the same one
B. rather different
C. a little different
D. doesn't mention