Answer: B.Heat is never converted completely into mechanical energy.
Explanation:
According to the second principle of thermodynamics:
"The amount of entropy in the universe tends to increase over time"
However the first formulation of this law (by Sadi Carnot) states:
"There is an upper limit to the efficiency of conversion of heat to work, in a heat engine
"
This means the heat cannot be completely transformed into mechanical energy in a machine. That is why a machine's effieciency is always less than 100%
