the average heat transfer cofficient for cube.

Question

 A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficient of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is

(IES-96)  

(a) h₁ + h₂ + h₃            (b) (h₁ h₂ h₃)^1/3

^{(c)$\frac{1}{h_1}+\frac{1}{h_2}+\frac{1}{h_3}$}

^{(d) none of the above}