Suppose that a market efficiency supporter was trying to explain the high stock market returns between the start of 2009 and 2017 (over 15% per annum, annualized). He argued that the stock market was not irrationally overvalued, but rather merely that investors had (rationally) realized that the stock market is not particularly risky, so that required returns on the stock market had fallen. Explain this argument using the present value framework (dividend discount model).