Consider a rectangular coordinate system (x, y, z) with unit vectors ax, ay, and az. A plane wave traveling in the region z ≥ 0 with electric field vector E = 10 cos (2 × 108 t + βz) ay is incident normally on the plane at z = 0, where β is the phase constant. The region z ≥ 0 is in free space and the region z < 0 is filled with a lossless medium (permittivity ε = ε0, permeability μ = 4μ0, where ε0 = 8.85 × 10-12 F/m and μ0 = 4π × 10-7 H/m). The value of the reflection coefficient is

• A. \(\dfrac{2}{3}\)
• B. \(\dfrac{3}{5}\)
• C. \(\dfrac{1}{3}\)
• D. \(\dfrac{2}{5}\)