Two parallel disks each having radius $R$ are separated by a distance $l$. The surface charge densities are $\sigma$ and $-\sigma$. The electric field at point $P$,a large distance $r$ along the axis of the disks is:

• A
  $\frac{{\sigma {R^2}l}}{{{\varepsilon _0}{r^3}}}$
• B
  $\frac{{2\sigma {R^2}l}}{{{\varepsilon _0}{r^3}}}$
• C
  $\frac{{\sigma {R^2}l}}{{4{\varepsilon _0}{r^3}}}$
• D
  $\frac{{\sigma {R^2}l}}{{2{\varepsilon _0}{r^3}}}$