A particle of mass $m$ and charge $q$ is thrown from origin at $t = 0$ with velocity $2\hat{i}$ + $3\hat{j}$ + $4\hat{k}$ units in a region with uniform magnetic field $\vec B$ = $2\hat{i}$ units. After time $t =\frac{{\pi m}}{{qB}}$ , an electric field  is switched on such that particle moves on a straight line with constant speed. $\vec E$ may be
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In time $\frac{\mathrm{T}}{2}=\frac{\pi \mathrm{m}}{\mathrm{qB}}$

$\mathrm{V}_{\perp}$ gets reversed

$\overrightarrow {\rm{v}}  = 2\widehat {\rm{i}} - 3\widehat {\rm{j}} - 4\widehat {\rm{k}}$

$\mathrm{q}(\overrightarrow{\mathrm{E}}+\overrightarrow{\mathrm{v}} \times \overrightarrow{\mathrm{B}})=0$

$\overrightarrow {\rm{E}}  =  - (\overrightarrow {\rm{v}}  \times \overrightarrow {\rm{B}} ) =  - (2\widehat {\rm{i}} - 3\widehat {\rm{j}} - 4\widehat {\rm{k}}) \times 2\widehat {\rm{i}} =  - 6\widehat {\rm{k}} + 8\widehat {\rm{j}}$

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