问题如下：
Which of the following choices correctly characterizes basis-point volatility and yield volatility as a function of time within the lognormal model?
选项：
Basis-point volatility
Yield volatility
increases
constant
increases
decreases
decreases
constant
decreases
decreases
解释：
Choices B and D can be eliminated because yield volatility is constant. Basis-point volatility under the CIR model increases at a decreasing rate, whereas basis-point volatility under the lognormal model increases linearly. Therefore, basis-point volatility is an increasing function for both models.
lognormal model的basis-point volatility等于σr，b-p volatility 应该是跟r成比例增加，为什么也会随着时间time 而增加呢？是默认时间越长，r也越大吗？
