问题如下：
Which of the following statements best characterize the Model 4 (Black-Karasinski model)?
选项：
A.
The model produces an interest rate tree that is recombining by definition.
B.
The model produces an interest rate tree that is recombining by adjusting the dt variable.
C.
The time-varying model has a slow speed of mean-reverting adjustment.
D.
The time-varying model has a fast speed of mean-reverting adjustment.
解释：
B is correct.
考点：Model 4 (Black-Karasinski model)
解析：
为了使得利率的节点变得重合，该模型中dt的取值（也就是每一次的时间间隔）是可以人为调整、认为设置成不一样的。
该模型对回归速度没有预测。
麻烦问下这里的model 4 是lognormal的模型嘛？
