这道题的解释没太理解，最长期的分层为什么有最小的缩期风险？
问题如下图：
选项：
A. 
B. 
C. 
解释：
NO.PZ2017012501000009 问题如下 The longest-term tranche of a sequential-pCMO is most likely to have the lowest: A.average life. B.extension risk. C.contraction risk. C is correct.For a CMO with multiple sequential-ptranches, the longest-term tranche will have the lowest contraction (prepayments greater thforecaste risk because of the protection against this risk offerethe other tranches. The longest-term tranche is likely to have the highest average life anextension risk because it is the last tranche repaiin a sequential-ptranche. 考点CMO解析longest term tranche是最后还款的那一层，该层的延期风险最大，缩期风险最小，故C正确。 老师这题错了五次！！除了support，其他都是先还上层吗？傻傻分不清，能不能帮忙捋一下~~
extension risk. contraction risk. C is correct. For a CMO with multiple sequential-ptranches, the longest-term tranche will have the lowest contraction (prepayments greater thforecaste risk because of the protection against this risk offerethe other tranches. The longest-term tranche is likely to have the highest average life anextension risk because it is the last tranche repaiin a sequential-ptranche. 时间分层time tranching，longest term tranche是最后还款的那一层，该层的延期风险最大，缩期风险最小。无法理解
老师，这题没太明白，the longest term tranche指的是the last tranche, 也就是相当于c那一层吗？
