问题如下：
Aria and Ben are discussing about time-dependent drift models.
Aria: Time-dependent drift models are flexible because volatility can change from period to period. And volatility must be an increasing function of short-term rate volatilities.
Ben: Time-dependent volatility functions are useful for pricing interest rate caps and floors.
Who is correct about the time-dependent drift models?
选项：
A.Aria only.
B.Ben only.
C.Both Aria and Ben.
D.Neither Aria nor Ben.
解释：
B is correct.
考点：Time-dependent drift model
解析：
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.
老师好，根据答案解析，A同学的statement，前半句volatility can change from period to period是对的吗？A同学错只是错在后半句，不但可以increasing，还可以decreasing或者是constant?
如果上面的理解正确的话，但是根据Ho-Lee Model的公式，volatility是不变的啊？
