Let a variable line passing through the centre of the circle \( x^{2}+y^{2}-16x-4y=0 \), meet the positive co-ordinate axes at the points \( A \) and \( B \). Then the minimum value of \( OA+OB \), where \( O \) is the origin, is equal to:
\( 18 \)
\( 12 \)
\( 20 \)
\( 24 \)
