问题如下：
The option value of a callable bond is equal to:
选项：
A.Z-spread minus I-spread.
B.Option-adjusted spread minus Z-spread.
C.Z-spread minus option-adjusted spread.
解释：
C is correct.
By definition, the option value of a callable bond is equal to Z-spread minus option-adjusted spread.
老师，请问一下这道题的原理是？谢谢！
