4. In a perfect dielectric medium with the permeability of \(\mu\), there exists an electric field given as \(\mathbf{E} = E₀cos(ωt - kz)ax, where E₀ is the peak value, \(\omega\) is the frequency, k is a constant, and \(\hat{a}_x\) is the unit vector in x direction. What is the magnetic field (H) in the region?
(A)
(B)0
(C)
(D)

(E)None of the above

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