8. John tried to find the relation between wave vector and its components kx, ky, kz of guided modes in a hollow metallic waveguide via v² = vx² + vy² + vz² (eq.1), where v² = , and is the velocity of a guided mode in the waveguide. He used the formula vi =, i = x, y, z (eq.2), and then obtained (eq.3), which is different from the usual understanding of k² = kx² + ky² + kz² (eq.4). Which of the following statement is correct: (A) The eq.1 is not correct. (B) The eq.2 is not valid for the guided modes in this waveguide. (C) The eq.4 is not valid for the guided modes in this waveguide. (D) None of the above