问题如下：
Regarding the validity of time-dependent drift models, which of the following statements is(are) correct?
i. Time-dependent drift models are flexible since volatility from period to period can change. However, volatility must be an increasing function of short-term rate volatilities.
ii. Time-dependent volatility functions are useful for pricing interest rate caps and floors.
选项：
A. I only.
B. II only.
C. Both I and II.
D. Neither I nor II.
解释：
Time-dependent volatility models are very flexible and can incorporate increasing, decreasing, and constant short-term rate volatilities between periods. This flexibility is useful for valuing interest rate caps and floors because there is a potential payout each period, so the flexibility of changing interest rates is more appropriate than applying a constant volatility model.
我想问一下解析里多钱“constant short term rate volatilities between periods”怎么理解呢？63题中，不是还提到V model的volatility是decresing的吗？解析所说的constant是指？
