A proposed Ocean Thermal-Energy Conversion (OTEC) system is a heat engine that would operate between warm water (17.2 degrees Celsius) at the ocean's surface and cooler water (6 degrees Celsius), 1720 m below the surface. What is the maximum possible (Carnot) efficiency of the system (express the answer as a percent, not a decimal, and don't enter the percent symbol)?

a) 50%
b) 60%
c) 70%
d) 80%