A sphere maintained at temperature 600 K, has cooling rate R in an external environment of 200 K temperature. If its temperature falls to 400 K, the cooling rate will be:

• A

  $\frac{3}{16}\mathrm{R}$

• B

  $\frac{16}{3}\mathrm{R}$

• C

  $\frac{9}{27}\mathrm{R}$

• D

  $\frac{3}{64}\mathrm{R}$