Based on the second law of thermodynamics why must a machine always be less than 100% efficient?
a. heat never moves from cold to hot.
b. heat is never converted completely into mechanical energy.
c. heat never flows from hot to cold.
d. entropy never increases.

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%

false

explanation:

boats, things on boats luggage are way more heavy then water.