问题如下：
The factor considered by rating agencies when a corporation has debt at both its parent holding company and operating subsidiaries is best referred to as:
选项：
A.credit migration risk.
B.corporate family rating.
C.structural subordination.
解释：
C is correct.
Structural subordination can arise when a corporation with a holding company structure has debt at both its parent holding company and operating subsidiaries. Debt at the operating subsidiaries is serviced by the cash flow and assets of the subsidiaries before funds are passed to the parent holding company.
请问A，B选项能帮忙解释一下嘛？谢谢
