$13.$ The following four issues are all option$-$free bonds; which has the greatest interest rate risk?

Issue Coupon Maturity Yield required by market

A $6\mathrm{.}50\%12$ years $7\%$

B $7\mathrm{.}25\%12$ years $7\mathrm{.}40\%$

C $6\mathrm{.}50\%12$ years $7\mathrm{.}20\%$

D $7\mathrm{.}50\%11$ years $8\%$