If $b$ and $c$ are the lengths of the segments of any focal chord of a parabola $y^2 = 4ax$,then the length of the semi-latus rectum is:

• A
  $\frac{bc}{b+c}$
• B
  $\sqrt{bc}$
• C
  $\frac{b+c}{2}$
• D
  $\frac{2bc}{b+c}$