When using a real machine to do work, input work will always be less than output work, because machines increase the amount of energy available. greater than output work, because energy is lost to the environment by the machine. equal to output work, because energy cannot be created or destroyed. different than output work, because the machine either creates or loses energy.

The correct answer is:

When using a real machine to do work, input work will always be greater than output work, because energy is lost to the environment by the machine.

Energy is lost by the machine in the environment through the process of heat and friction. This is one of the many obvious laws of physics that output work always entails the loss of energy in the form of heat one way or another. 

That would be a working theory.