To convert ocean thermal energy into electricity, it is necessary that a difference of more than _____ between the surface waters and water depths of 1000m is required.

Question

$\begin{array}{1 1} (a)\; 10^{\circ} \\ (b)\; 20^{\circ} \\ (c)\; 25^{\circ} \\ (d)\; 30^{\circ} \end{array}$