Use the data provided on Cadbury to answer the question below. The risk$-$free rate is $4\mathrm{.}25\%.$ The expected return on the market portfolio is $9\mathrm{.}75\%.$ The corporate tax rate is $40\%.$ The face value of Cadbury's outstanding bonds is $2\mathrm{.}450$ billion pounds sterling. The coupon rate on Cadbury's bonds is $4\mathrm{.}5\%.$ Assume that the bonds pay annual coupons. The yield to maturity on Cadbury's bonds is $4\mathrm{.}5\%.$ Cadbury's bonds mature in $7$ years. Cadbury has $1\mathrm{.}650$ billion common shares outstanding. The market price of Cadbury's common shares as of Dec $31,2008$ is $6\mathrm{.}25$ pounds sterling. Cadbury's Beta is $0\mathrm{.}8.$ What is Cadbury's cost of debt $($after$-$tax$)?$

$2\mathrm{.}70\%$

$8\mathrm{.}70\%$

$7\mathrm{.}80\%$

$8\mathrm{.}65\%$

$4\mathrm{.}50\%$