NO.PZ2018123101000116
问题如下：
Which of the following statements comparing the Ho–Lee and Kalotay–Williams–Fabozzi (KWF) equilibrium term structure models is correct?
选项：
A.The Ho–Lee model assumes constant volatility, while the KWF model does not.
B.The KWF model incorporates the possibility of negative rates, while the Ho–Lee model does not.
C.The KWF model describes the log of the dynamics of the short rate, while the Ho–Lee model does not.
解释：
C is correct. The Kalotay–Williams–Fabozzi equilibrium term structure model is similar to the Ho–Lee model in that it assumes constant drift, no mean reversion, and constant volatility, but the KWF model describes the log of the dynamics of the short rate, while the Ho–Lee model does not.
答案说“The Kalotay–Williams–Fabozzi equilibrium term structure model is similar to the Ho–Lee model in that it assumes constant drift, no mean reversion” 说"it assumes constant drift" , 这是为什么？不是volatility才是constant，然后drift term是time dependent么
