$3.$ Suppose a panel of economists predicts that a nation's real GDP per capita will double in approximately $20$ years. Based upon the rule of $70,$ what must be the predicted annual growth rate of real GDP per capita?a$.$ MACROBUTTON HTMLDirect $140\%$ b$.$ MACROBUTTON HTMLDirect $3\mathrm{.}5\%$ c$.$ MACROBUTTON HTMLDirect $2\mathrm{.}85\%$ d$.$ MACROBUTTON HTMLDirect $14\%$