Circular polarised wave

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In this I could not understand why D option is wrong .
As both dy/dt and dz/dt are dependent on x and time

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The particle speed $v=\sqrt{\dot y^2+\dot z^2}$.
$\dot y=-\omega a \cos\phi$
$\dot z=+\omega a\sin\phi$
where $\phi=kx-\omega t$.
So although $\dot y, \dot z$ each depend on $x, t$ the combination $v=\sqrt{\dot y^2+\dot z^2}=\omega a=$ constant does not depend on either $x$ or $t$.