The temperature difference between two sides of a metal plate,$3 \ cm$ thick,is $15^{\circ} C$. Heat is transmitted through the plate at the rate of $900 \ kcal$ per minute per $m^2$ at steady state. The thermal conductivity of the metal is:

• A
  $1.8 \times 10^{-2} \ \frac{kcal}{m \cdot s \cdot ^{\circ}C}$
• B
  $4.5 \times 10^{-2} \ \frac{kcal}{m \cdot s \cdot ^{\circ}C}$
• C
  $3 \times 10^{-2} \ \frac{kcal}{m \cdot s \cdot ^{\circ}C}$
• D
  $6 \times 10^{-2} \ \frac{kcal}{m \cdot s \cdot ^{\circ}C}$