Heat of combustion of $CH_4(g), \: C(graphite),\: H_2(g)$ are -20 kcal, -40 kcal and -10 kcal respectively. The heat of formation of methane is

Question

$\begin {array} {1 1} (A)\;-40 \: kcal/mol & \quad (B)\;40\: kcal/mol \\ (C)\;-80\: kcal/mol & \quad (D)\;80\: kcal/mol \end {array}$