A firm has $$54$ billion in equity, $$58$ billion in debt, and a tax rate of $21\%.$ The pre$-$tax cost of debt and equity are $1\mathrm{.}9\%.$ and $9\mathrm{.}7\%.,$ respectively. What discount rate should it use for a typical project?

$4\%$

$8\%$

$5\%$

$9\%$