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

A. No system can convert energy from one form to another useful
form with 100% efficiency.

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

C. Entropy of a system increases over time.

D. It's possible to change heat completely into work, with no other
change taking place.