问题如下：
The single monthlymortality rate (SMM) most likely:
选项：
A. increases as extension riskrises.
B. decreases as contractionrisk falls.
C. stays fixed over time whenthe standard prepayment model remains at 100 PSA.
解释：
B is correct.The SMM is a monthly measure of the prepayment rate or prepayment speed.Contraction risk is the risk that when interest rates decline, actual prepayments will be higher than forecast. Soif contraction risk falls, prepayments are likely to be lower than forecast,which would imply a decrease in the SMM.
A isincorrect because the SMM is a monthly measure of the prepayment rate or prepayment speed. Extension risk is therisk that when interest rates rise, actual prepayments will be lower thanforecast. So if extension risk rises, prepayments are likely to be lower thanforecast, which would imply a decrease, not an increase, in the SMM.
C is incorrect because at 100 PSA, investors canexpect prepayments to follow the PSA prepayment benchmark. Based on historical patterns, thePSA standard model assumes that prepayment rates are low for newly initiatedmortgages and then speed up as mortgages season. Thus, 100 PSA does not implythat the SMM remains the same but, rather, implies that it willvary over the life of the mortgage.
请问C里不是按照0.2%固定吗？能具体解释一下吗？好像有点混乱了
