Two identical charges $+Q$ are kept fixed at 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.