The "Kangri" is an earthen pot used to stay warm in Kashmir during the winter months. Assume that the "Kangri" is spherical and of surface area $7\times {10}^{-2} {\mathrm{m}}^2.$ It contains $300 \mathrm{g}$ of a mixture of coal, wood and leaves with calorific value of $30 \mathrm{kJ} {\mathrm{g}}^{-1}$ (and provides heat with $10\%$ efficiency). The surface temperature of the 'Kangri' is $60°\mathrm{C}$ and the room temperature is $0°\mathrm{C}.$ Then, a reasonable estimate for the duration t (in hours) that the 'kangri' heat will last is (take the 'kangri' to be a black body):

• A

  $8$
• Correct answer
  B

  $10$
• C

  $12$
• D

  $16$

Hence, the correct answer is option: 'B'.