Four point positive charges of same magnitude $(Q)$ are placed at four corners of a rigid square frame of side $L$ as shown in the figure. The plane of the frame is perpendicular to the $Z$-axis. If a negative point charge $(-q)$ is placed at a small distance $z$ away from the center of the frame along the $Z$-axis $(z < < L)$, then:

• A
  The negative charge oscillates along the $Z$-axis.
• B
  It moves away from the frame.
• C
  It moves slowly towards the frame and stays in the plane of the frame.
• D
  It passes through the frame only once.