A small sphere of mass $m =\ 0.5\, kg$ carrying a positive charge $q = 110\  \mu C$ is connected with a light, flexible and inextensible string of length $r = 60 \ cm$ and whirled in a vertical circle. If a vertically upwards electric field of strength $E = 10^5 NC^{-1}$ exists in the space, The minimum velocity of  sphere required at highest point so that it may just complete the circle........$m/s$  $(g = 10\, ms^{-2})$

  • A

    $8$

  • B

    $7$

  • C

    $6$

  • D

    $9$

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