What is the main idea of the first law of thermodynamics? A. Energy can be created or destroyed, but it can also change forms. B. Energy can be created but not destroyed. C. Energy can be destroyed bu not created. D. Energy can't be created or destroyed, it just changes forms.

The correct answer is D. The main idea of the first law of thermodynamics is that energy cannot be created or destroyed, it just changes forms. This law, also known as the law of conservation of energy, states that the total amount of energy in a closed system remains constant over time. Energy can be transferred from one form to another, such as from kinetic energy to potential energy, but the total energy remains the same.