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 single monthly mortality rate (SMM) most likely:
Option A:increases as extension risk rises.
Option B:decreases as contraction risk falls.
Option C:stays fixed over time when the standard prepayment model remains at 100 PSA.

Answer: B