什么叫mortgage pass-through security？
问题如下图：
选项：
A. 
B. 
C. 
解释：
NO.PZ2017012501000007 问题如下 If interest rates increase, investor who owns a mortgage pass-through security is most likely affecteby: A.cret risk. B.extension risk. C.contraction risk. B is correct.Extension risk is the risk thwhen interest rate rise, fewer prepayments will occur. Homeowners will reluctant to give up the benefit of a contractuinterest rate this lower. a result, the mortgage passthrough security becomes longer in maturity thanticipatethe time of purchase. 考点延期风险解析当利率上升时，homeowner不会选择提前还款，这样一来导致的风险就是extension risk，即延期风险，故B正确。 如题
为什么extension 呢？r上升，为啥呢
