$A$ particle of mass $m$ and charge $q$ is fastened to one end $A$ of a massless string having equilibrium length $\ell$,whose other end is fixed at point $O$. The whole system is placed on a frictionless horizontal plane and is initially at rest. If a uniform electric field $E$ is switched on along the $x$-direction as shown in the figure,then the speed of the particle when it crosses the $x$-axis is

  • A
    $\sqrt{\frac{2 qE \ell}{m}}$
  • B
    $\sqrt{\frac{q E \ell}{4 m}}$
  • C
    $\sqrt{\frac{q E \ell}{m}}$
  • D
    $\sqrt{\frac{q E \ell}{2 m}}$

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