= The motion of a charged particle in a constant $\vec{B}$ =

  A. is time-reversal invariant
  A. is not time-reversal invariant

This question is an advanced level question.  But it is easy to answer, if one keeps in mind the following definition.  '''Time-reversal invariance''': if any possible motion played backwards is also a possible motion, then the system is time-reversal invariant.

Ans: B (Note that the system is taken as the charged particle only.  See LN for more discussion.)

= The motion of a charged particle in a constant $\vec{B}$ =

  A. conserves the mechanical energy
  A. does not conserve the mechanical energy

Ans: A

= The work energy "theorem" =

The work energy theorem, $\Delta T = W$ where $T$ is the kinetic energy (and $\Delta T = T_2 - T_1$ is its change) and $W$ is the net work done on the particle, is

  A. always valid (in classical mechanics).
  A. valid only for conservative forces.

Ans: A (This is how the kinetic energy is ''defined''!)