Six point charges are placed at the vertices of a regular hexagon of side $a$ as shown in the figure. Three charges are $+Q$ and three are $-Q$ arranged alternately. The electric field intensity at a point on the line passing through the centre $O$ and perpendicular to the plane of the figure at a large distance $x (x \gg a)$ from $O$ is (Let $\frac{1}{4 \pi \epsilon_0} = k$):

• A
  $k \times \frac{4 Q a}{x^3}$
• B
  $k \times \frac{2 Q a}{x^3}$
• C
  $k \times \frac{8 Q a}{x^3}$
• D
  $0$