Hi, I have a question in Srednicki's QFT textbook. In p.460 section 75(about Chiral gauge theory), it says "In spinor electrodynamics, the fact that the vector potential is odd under charge conjugation implies that the sum of these diagrams(exact 3photon vertex at one-loop) must vanish." That's good, because it's just the Furry's theorem for odd number of external photons. But I find the subsequent statement confusing. "For the present case of a single Weyl field(coupled to U(1) field), there is no charge conjugation symmetry, and so we must evaluate these diagrams." But shouldn't the transformation rule of [itex]A^\mu (x)[/itex] under C,P or T be universal regardless of specific theory?