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 CDO tranche with a credit rating status between senior and subordinated bond classes is called the:
Option A:equity tranche.
Option B:residual tranche.
Option C:mezzanine tranche.

Answer: C