Temperature gradient dT/dL at the centre of the bar is

Question

 A metallic rod of uniform diameter and length L connects two heat sources each at 500⁰C.

The atmospheric temperature is 30⁰C. The temperature gradient dT/dL at the centre of the

bar will be

(a) $\frac{500}{L/2}$      (b) -$\frac{500}{L/2}$        (c) $\frac{470}{L/2}$      (d) Zero