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.

An option-adjusted spread (OAS) on a callable bond is the Z-spread:
Option A:over the benchmark spot curve.
Option B:minus the standard swap rate in that currency of the same tenor.
Option C:minus the value of the embedded call option expressed in basis points per year.

Answer: C