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 key to a CDO’s viability is the creation of a structure with a competitive return for the:
Option A:senior tranche.
Option B:mezzanine tranche.
Option C:subordinated tranche.

Answer: C