For which one of the following reactions is the value of $H{?}_{xn}$ equal to $H_f$ for the product? I$)2C($s$,$ graphite$)+2H_2($g$)C_2{H}_4($g$)$ II$)N_2($g$)+O_2($g$)2NO($g$)$ III$)2H_2($g$)+O_2($g$)2H_{2}O($l$)$ IV$)2H_2($g$)+O_2($g$)2H_{2}O($g$)$ V$)H_{2}O($l$)+\frac{1}{2}{O}_2($g$)$