Two charged particles traverse identical helical

Question:
Two charged particles traverse identical helical paths in an opposite sense in a uniform magnetic field

$B=B_{0} \hat{k}$

(a) they have equal z-components of momenta

(b) they must have equal charges

(c) they necessarily represent a particle-antiparticle pair

(d) the charge to mass ratio satisfy: (e/m)1 + (e/m)2 = 0

Solution:

(d) the charge to mass ratio satisfy: (e/m)1 + (e/m)2 = 0