From the closing of December 31, 2016 to December 31, 2021, Vanguards S&P 500 VFIAX fund, which tracks the S&P500 index, returned the following annual rates of return: 2017=21.8%; 2018=-4.4%; 2019=31.5%; 2020=18.4%; 2021=28.7% What was the rate of return over the first 3 years