Which of the Following Is the Main Reason for the Fact

Which of the following is the main reason for the fact that hydroelectric power plants are far more efficient than coal- fired electric power plants?

A) Hydroelectric plants have far fewer moving parts.
B) Coal is only partially combustible, due to the presence of large amounts of impurities.
C) Since the hydroelectric plant is not a heat engine, it is not restricted by the second law of thermodynamics.
D) The law of conservation of energy tells us that heat engines, such as hydroelectric plants, are more efficient than other forms of energy conversion.
E) Actually, hydroelectric plants are not significantly more efficient than coal- fired plants.