support tranche和PAC到底有什么差别啊？这题可以再具体解释一下吗？

问题如下：

Considera collateralized mortgage obligation (CMO) structure with one plannedamortization class (PAC) tranche and one support tranche outstanding. Also, assume that theprepayment speed is higher than the upper collar on the PAC. Which of the following statementsis most accurate? The:

选项：

A.PAC tranche has no risk of prepayments. B.Average life of the support tranche will contract. C.Average life of the PAC tranche will extend. D.Average life of the support tranche will extend.

解释：

If the prepayment speed is faster than the PAC collar, the support tranche receives a higher level of prepayments (so that the PAC tranche remains at the upper collar of the PAC). The average life of the support tranche will contract (shorten). The PAC tranche could receive higher prepayments if eventually the support tranche is fully repaid its principal (i.e., a busted PAC). However, the question says that the support tranche is still outstanding, which means that hasn’t happened yet.