问题如下：
The variability of the coupon rate on a Libor-based floating-rate bond is most likely due to:
选项：
A.periodic resets of the reference rate.
B.market-based reassessments of the issuer’s creditworthiness.
C.changing estimates by the Libor administrator of borrowing capacity.
解释：
A is correct.
Changes in the coupon rate of interest on a floating-rate bond that uses a Libor reference rate are due to changes in the reference rate (for example, 90-day Libor), which resets periodically. "Therefore, the coupon rate adjusts to the level of market interest rates (plus the spread) each time the reference rate is reset."
请解释一下B和C为什么不对，公司信用变化不会影响spread的变化从而影响coupon rate吗？还有C，是怎么理解的呢？
