13. The electric field of an electromagnetic wave is written as E(x, y, z, t) = E₀ cos(ky - ωt) eₓ, where E₀ is the amplitude, eₓ is the unit vector of z-axis, k is the wave number, and ω is the angular frequency. What is the magnetic field of this wave? (A) B(x, y, z, t) = (E₀/c) cos(ky - ωt) eₓ (B) B(x, y, z, t) = -c E₀ cos(ky - ωt) eₓ (C) B(x, y, z, t) = -(E₀/c) cos(ky - ωt) eᵧ (D) B(x, y, z, t) = (E₀/c) cos(kz - ωt) eₓ (E) B(x, y, z, t) = c E₀ cos(kx - ωt) eᵧ