9. A standing light wave is composed by two counter-propagating plane waves. The electric field is written as Estanding(r, t) = E₁(r, t) + E₂(r, t), where E₁(r, t) = E₀ cos(kz − ωt) ex and E₂(r, t) = E₀ cos(−kz − ωt) ex. Find the magnetic field of the standing wave.
(A) B(r, t) = (2E₀/c) cos(kz) cos(ωt) ez
(B) B(r, t) = (2E₀/c) cos(kz) cos(ωt) ey
(C) B(r, t) = (2E₀/c) sin(kz) cos(ωt) ey
(D) B(r, t) = (2E₀/c) cos(kz) sin(ωt) ey

(E) B(r, t) = (2E₀/c) sin(kz) sin(ωt) ey

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