The temperature difference between two sides of an iron plate,$1.8 \ cm$ thick is $9^{\circ} C$. Heat is transmitted through the plate at a rate of $10 \ kcal / (s \cdot m^2)$ at steady state. The thermal conductivity of iron is:

• A
  $0.02 \ kcal / (m \cdot s \cdot ^{\circ} C)$
• B
  $0.04 \ kcal / (m \cdot s \cdot ^{\circ} C)$
• C
  $0.05 \ kcal / (m \cdot s \cdot ^{\circ} C)$
• D
  $0.004 \ kcal / (m \cdot s \cdot ^{\circ} C)$