A sphere of surface area $4 {\mathrm{m}}^2$ at temperature $400 \mathrm{K}$ and having emissivity $0.5$ is located in an environment of temperature $200 \mathrm{K}$. The net rate of energy exchange of the sphere is

(Stefan Boltzmann constant $\sigma =5.67\times {10}^{-8} \mathrm{W} {\mathrm{m}}^{-2} {\mathrm{K}}^4$)