问题如下：
The tranches in a collateralized mortgage obligation (CMO) that are most likely to provide protection for investors against both extension and contraction risk are:
选项：
A.planned amortization class (PAC) tranches
B.support tranches.
C.sequential-pay tranches.
解释：
A is correct.
PAC tranches have limited (but not complete) protection against both extension risk and contraction risk. This protection is provided by the support tranches. A sequential-pay tranche can protect against either extension risk or contraction risk but not both of these risks. The CMO structure with sequential-pay tranches allows investors concerned about extension risk to invest in shorter-term tranches and those concerned about contraction risk to invest in the longer-term tranches.
为什么不是B,他不是保护了PAC吗
