Given a text T, and several options, according to the question posed in the text T to choose the most appropriate option from the options as the answer, please directly output the most appropriate option, do not need to output the explanation.

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:
Option A:credit migration risk.
Option B:corporate family rating.
Option C:structural subordination.

Answer: C