If $1.2\times {10}^6\mathrm{J}$ of heat energy is given off in $1\sec$ from a vessel maintained at a temperature gradient of $30{\mathrm{km}}^{-1}$, the surface area of the vessel is ____________

[Thermal capacity of the vessel $=$ $400{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ ]

A. $1.0\times {10}^3{\mathrm{m}}^2$  
B. $1.0\times {10}^2{\mathrm{m}}^2$
C. $9.0\times {10}^4{\mathrm{m}}^2$
D. $9.0\times {10}^2{\mathrm{m}}^2$

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Correct Answer: A