问题如下图：
选项：
A. 
B. 
C. 
D. 
解释：
答案是c吗？
答案里可不可以写一个正确 有时候看不太明白的时候 还要去猜到底哪个是正确答案 所以应该是选b吗这道题
It stays constant over time. It generally tracks nominrates over time. It is volatile over time, similto both reannominrate. It shoulpointeout thwhile it is true ththe regression hee assumes a constant betthis is not a realistic assumption; thus, it is best to estimate beta over severtime perio ancompare accorngly. 讲义里好像没有提到β是恒定不变的？（如果有，麻烦老师告知在哪个地方）这是结论么？
这道题目是考的哪个知识点？
不变的话如何provi estimate of the volatility of hee ratio？是不是一段时间不变而已？
C为什么不对？
