[单选题]
The total mechanical en+ 2ltf;2oqc snergy ppai2f g(bhwb. z,f)7nf5 7sak sx2a;of a system
A. is equally divided between kinetic energy and potential energy.
B. is either all kinetic energy or all potential energy, at any one instant.
C. can never be negative.
D. is constant, only if conservative forces act.
