[求助]下午mock104题？

104 A fixed income portfolio manager is revaluating investments in the mortgage market but is concerned about prepayment risk. The security that will most likely minimize prepayment risk is

A a mortgage passthrough security

B a portfolio of interest-only mortgage loans

c tranche B of a collateralized mortgage obligation

答案是C 。我想问的是答案B的 interest-only mortgage loans 是什么意思呢 是说在到期前只付interest吗 如果那样可能B还好点 。C答案在trancheA的本金支付完之后 也会开始支付本金的，还是有不少prepayment risk啊 。。。求哪位大侠指点下。。