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A plane electromagnetic wave of the form $\vec{E}=\hat{y} E_{0}\left[\cos 2 \pi\left(5 \times 10^{14} \sec ^{-1}\right) t-\left(2.5 \times 10^{6} \mathrm{~m}^{-1}\right) x\right]$ (where $E_{0}$ is a constant and $y$ is the unit vector along $y$-direction) represents a wave propagating along

- $+x$ direction
- $+y$ direction
- $-x$ direction
- $-y$ direction