Consider zero$-$coupon bonds with a face value of $$1,000$ and time to maturity of $6$ years. The bonds are currently trading at $$640.$ What is this bond's yield to maturity?Oa$\mathrm{.}5\mathrm{.}7\%06\mathrm{.}6\mathrm{.}6\%$Oc$77404\mathrm{.}93\%$Oe$\mathrm{.}11\mathrm{.}8\%$