Heat is transferred steadily through a $0.2-\mathrm{m}$ thick $8 \mathrm{m} \times 4 \mathrm{m}$ wall at a rate of $2.4 \mathrm{kW}$. The inner and outer surface temperatures of the wall are measured to be $15^{\circ} \mathrm{C}$ and $5^{\circ} \mathrm{C} .$ The average thermal conductivity of the wall is
$(a) 0.002 \mathrm{W} / \mathrm{m} \cdot^{\circ} \mathrm{C}$
$(b)0.75 \mathrm{W} / \mathrm{m} \cdot^{\circ} \mathrm{C}$
$(c) 1.0 \mathrm{W} / \mathrm{m} \cdot^{\circ} \mathrm{C}$
$(d) 1.5 \mathrm{W} / \mathrm{m} \cdot^{\circ} \mathrm{C}$
$(e) 3.0 \mathrm{W} / \mathrm{m} \cdot^{\circ} \mathrm{C}$