问题如下：
A senior unsecured credit instrument holds a higher priority of claims than one ranked as:
选项：
A.mortgage debt.
B.second lien loan.
C.senior subordinated.
解释：
C is correct.
The highest-ranked unsecured debt is senior unsecured debt. Lower-ranked debt includes senior subordinated debt. A and B are incorrect because mortgage debt and second lien loans are secured and higher ranked.
mortgage debt是属于哪种呢？
