PAC是给investor提供保护了，那support是给谁提供保护了呢，给PAC么。

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：

planned amortization class (PAC) tranches

support tranches.

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.

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，故选项A正确。