A positively charged thin metal ring of radius R is fixed in the xy plane with its centre at the origin O. A negatively charged particle P is released from rest at the point (0,0,z0) where z0>0. Then the motion of P is
a. periodic, for all values of z0 satisfying 0<z0<∞
b. simple harmonic, for all values of z0 satisfying 0<z0≤R
c. approximately simple harmonic, provided z0≪R
d. such that P crosses O and continues to move along the negative z axis towards z=−∞