Which Statement Below Regarding the First Law of Thermodynamics Is

Which statement below regarding the first law of thermodynamics is most correct?

A) A system can do work externally only if its internal energy decreases.
B) The internal energy of a system that interacts with its environment must change.
C) No matter what other interactions take place, the internal energy must change if a system undergoes a heat transfer.
D) The only changes that can occur in the internal energy of a system are those produced by non-mechanical forces.
E) The internal energy of a system cannot change if the heat transferred to the system is equal to the work done by the system.