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The magnetic field intensity vector of a plane wave is given by
$\overline{\mathrm{H}}(x, y, z, t)=10 \sin \left(50000 t+0.004 x+30 \; \hat{a}_{y}\right)$,
where $\hat{a}_{y}$ denotes the unit vector in $y$ direction.
The wave is propagating with a phase velocity

1. $5 \times 10^{4} \mathrm{~m} / \mathrm{s}$.
2. $-3 \times 10^{8} \mathrm{~m} / \mathrm{s}$.
3. $-1.25 \times 10^{7} \mathrm{~m} / \mathrm{s}$.
4. $3 \times 10^{8} \mathrm{~m} / \mathrm{s}$.