The Ocean Thermal Energy Conversion Project Uses the Surface Water

The ocean thermal energy conversion project uses the surface water near tropical islands with a temperature of 20°C as the hot temperature reservoir,and the water at some depth,with a temperature of 5.0°C,as the cold temperature reservoir for a thermodynamic engine.What is the maximum possible efficiency of an engine running between those two temperatures?

A) 4.7%
B) 5.1%
C) 7.9%
D) 15%
E) 30%