State whether the following statements are true or false, always justifying. a) The electromagnetic force preserves the chirality of the bi-spinors, but not necessarily the helicity. b) The helicity of a particle never changes in an interaction. c) In the presence of a weak magnetic field one can, with a good approximation, say that the spin commutes with the Hamiltonian. d) The helicity eingen-states for photons correspond to circular polarization