问题如下图：
选项：
A. 
B. 
C. 
解释：
B哪里错了？tranching不是也可以有信用分层么？
NO.PZ2016031001000097 问题如下 In a securitization, time tranching provis investors with the ability to choose between: A.extension ancontraction risks. B.senior ansubornatebonclasses. C.fully amortizing anpartially amortizing loans. A is correct.Time tranching is the process in whia set of bonclasses or tranches is createthallow investors a choiin the type of prepayment risk, extension or contraction, ththey prefer to bear. Senior ansubornatebonclasses are usein cret tranching. Cret tranching structures allow investors to choose the amount of cret risk ththey prefer to bear. Fully anpartially amortizing loans are two types of amortizing loans. 考点time tranching解析时间分层，可以使投资者在contraction risk和extension risk之间做选择。contraction risk是缩期风险，extension risk是延期风险。这两个风险一般是指在证券化的产品里，因为贷款的提前偿还速度和之前预设的提前还款速度不一致导致的。如果借款者可以提前还款，那么产生的是contraction risk； 如果借款者拖延还款，产生的是extension risk。故A正确。 如题
NO.PZ2016031001000097 如何理解这道题A的extension和contraction
C为什么错？A怎么理解呢？上课的时候只表示了ABC，那么extension 与 contraction 怎么对应ABC？
