Which of the following are formulations of the second law of thermodynamics?

A. It isn't possible to transfer heat from an object at a lower temperature to another object at a higher temperature unless work is done.

B. All systems can convert energy from one form to another useful form with 100% efficiency.

C. It isn't possible to change heat completely into work, with no other change taking place.

D. Entropy of a system decreases over time.