问题如下：
3. Based on the historical record of surprises in inflation and productivity, the historical equity risk premium for the US equity market, if it is used asan estimate of the forward-looking equity risk premium, should most likely be:
选项：
A.left unchanged.
B.adjusted upward.
C.adjusted downward.
解释：
C is correct.
A string of favorable inflation and productivity surprises may result in a series of high returns that increase the historical mean estimate of the equity risk premium. To mitigate that concern, the analyst may adjust the historical estimate downward based on an independent forward-looking estimate.
我选的是B，是这么考虑的。
有favorable inflation &productivity代表经济好，经济好时rm大，但是rf也大，所以ERP偏小的。
但没有favorable inflation &productivity后，ERP走阔，因此是upward adjust。
请问这样为什么是不对的呢？
