请问老师,第二题,感觉答案在解释c,但是却给了a答案,不知道到底是什么答案?同时,我想问一下,为什么答案d不可以,因为fat tail is most likely the result of the volatility and mean of dist changing over time. 但是在unconditional dist 下, mean and sd are the same for asset returns for.any given days.这不是可以理解unconditional dist不太会有fat tail吗?谢谢