Consider zero$-$coupon bonds with a face value of $$1,000$ and time to maturity of $8$ years. The bonds are currently trading at $$640.$ What is this bonds yield to maturity?

a$.5\mathrm{.}7\%$

b$.6\mathrm{.}6\%$

c$.7\mathrm{.}7\%$

d$.9\mathrm{.}3\%$

e$.11\mathrm{.}8\%$