可以解释一下C选项是什么吗？
问题如下图：
选项：
A. 
B. 
C. 
解释：
NO.PZ2017012501000010 问题如下 The tranches in a collateralizemortgage obligation (CMO) thare most likely to provi protection for investors against both extension ancontraction risk are: A.planneamortization class (PAtranches B.support tranches. C.sequential-ptranches. A is correct.Ptranches have limite(but not complete) protection against both extension risk ancontraction risk. This protection is provithe support tranches. A sequential-ptranche cproteagainst either extension risk or contraction risk but not both of these risks. The CMO structure with sequential-ptranches allows investors concerneabout extension risk to invest in shorter-term tranches anthose concerneabout contraction risk to invest in the longer-term tranches. 考点CMO解析support tranches承担了所有的风险，当本金偿还快于预期，support tranches会吸收掉所有超过预期的偿还部分；当本金偿还慢于预期，support tranches不接受任何本金偿还，全部让给PAC结构。因此，PAC是受到保护的那一个，它的两种风险都是最小的，故A正确。 我读题干的意思说的不就是support tranch的意思么
这题在经典题中，但是少了for investor，是不是应该选B。答案是不是有问题
为什么不是B,他不是保护了PAC吗
PAC的性质是啥？啥知识点，为啥能同时缩小两种risk?
