B, the electric field force of protons has nothing to do with the speed direction, so it is impossible to judge whether protons move from M to N or from N to M, so B is wrong;
C, d, because ao > ob, φM is greater than φ n according to the distribution characteristics of electric field lines and equipotential surfaces with the same amount of different point charges;
The potential of positive charge is large at the position with high potential, so the potential of proton at point M is greater than that at point N; So, C is wrong and D is right;
Therefore: ad.