Sorry to answer you so late but that would be truth because that is the first rule of science which is what thermodynamics depends in.
The first law of thermodynamics is a type of law of conservation but with thermodynamics systems.
So, in gogle I found this: "The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed."
Like I said, sorry to reply so late but I hope this helped! =)
