Two identical charges $+Q $ are kept fixed some distance apart. A small particle $P $ with charge $q$ is placed midway between them. If $P$  is given a small displacement $\Delta$ , it will undergo simple harmonic motion if

  • A

    $q$ is positive and $\Delta$ is along the line joining the charges.

  • B

    $q$ is positive and $\Delta$ is perpendicular to the line joining the charges.

  • C

    $q$ is negative and $\Delta$ is perpendicular to the line joining the charges.

  • D

    $A$ and $C$ both

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