The length of the latus rectum of a parabola, whose vertex and focus are on the positive \( x \)-axis at a distance \( R \) and \( S \) (\( > R \)) respectively from the origin, is:
\( 4(S-R) \)
\( 2(S+R) \)
\( 4(S+R) \)
\( 2(S-R) \)