What is a defining characteristic of energy? 

A)  It is used over and over again. 
B)  It tends to be concentrated by living organisms. 
C)  It recycles through the ecosystem. 
D)  It flows in only one direction.

Energy flows in a one-way direction from a high-energy state to a low-energy state, known as the law of thermodynamics. This means that energy cannot be recycled or used over and over again without some loss of energy in the process.