问题如下：
In securitization, what technique gives investors choice to distinguish extension and contraction risk?
选项：
A. Time tranching
B. Senior/subordinated structure
C. Credit tranching
解释：
A is correct.
Both senior/subordinated structure and credit tranching redistrubute credit risk.
Time tranching redistributes prepayment risk.
请问题目翻译过来是什么意思
