[单选题]
The total mechanical entj,g:j/. s5jxp3zdyr er pj wo/ )*j,j/zl*u-ffqvbx3cr7 lh. xgy 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.
