15. The magnetic field of an electromagnetic wave is written as B(x, y, z, t) = B₀ ex cos(kz - ωt), where B₀ is the amplitude, ex is the unit vector along x-axis, k is the wave number, and ω is the angular frequency. What is the electric field of this wave? (A) E(x, y, z, t) = -cB₀ ey cos(kz - ωt) (B) E(x, y, z, t) = cB₀ ez sin(ky - ωt) (C) E(x, y, z, t) = -(B₀/c) ez cos(kz - ωt) (D) E(x, y, z, t) = cB₀ ey sin(kz - ωt) (E) E(x, y, z, t) = (B₀/c) ey cos(ky - ωt) where c is the speed of light, e_y and e_z are the unit vectors along y-axis and z-axis respectively.