问题如下：
A commercial mortgage-backed security (CMBS) does not meet the debt-to-service coverage at the loan level necessary to achieve a desired credit rating. Which of the following features would most likely improve the credit rating of the CMBS?
选项：
A. Subordination
B. Call protection
C. Balloon payments
解释：
A is correct.
If specific ratios of debt to service coverage are needed, and those ratios cannot be met at the loan level, subordination is used to achieve the desired credit rating. Call protection protects investors against prepayment risk. Balloon payments increase the risk of the underlying loans.
a选项内部信用增级不是只是各个分层不一样但是总体并没有改变吗？
