64.依照 RBE(relative biological effectiveness)的大小,下列輻射由大到小依序排列的順序,何者正確? ① 60Co γ-ray ②250 kV X-ray ③150 MeV proton ④14 MeV neutron(A)①②③④(B)④③②①(C)①③②④(D)④②③①