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Which of the following electric field wave equations is NOT possible for an electromagnetic wave?

${ E }_{ y }=4.0cos[2\pi (\cfrac { t }{ { 10 }^{ 6 }s } \pm \cfrac { x }{ 300m })] $

${ E }_{ y }=2.5cos[2\pi (\cfrac { t }{ { 10 }^{ -6 }s } \pm \cfrac { x }{ 300m } )]$

${ E }_{ y }=1.6cos[2\pi (\cfrac { t }{ { 6.67\cdot 10 }^{ -7 }s } \pm \cfrac { x }{ 200m } )]$

${ E }_{ y }=0.9cos[2\pi (\cfrac { t }{ { 1.33\cdot 10 }^{ -6 }s } \pm \cfrac { x }{ 400m } )]$