This standby letter of credit is available by sight payment. We engage with you that upon receipt of your draft(s) and your signed statement or tested telex statement or SWIFT stating that the amount in USD represents the unpaid balance of indebtedness due to you by the borrower, we will pay you within 7 banking days the amount specified in your statement or SWIFT. All drafts drawn hereunder must be marked “drawn under XYZ Bank standby letter of credit No. 810LC040000027D dated 20 DECEMBER 2004”.这段话主要体现了备用证的哪个性质？

A. 不可撤销性
B. 独立性
C. 跟单性
D. 约束性