The length of sub tangent, ordinate and subnormal to the parabola $y^2=4ax$ at a point (different from origin) are in

Clearly length of sub tangent $=\left|y\left(\frac{dx}{dy}\right)\right|=A$
Length of ordinate  $=\left|y\right|=B$
Length of subnormal  $=\left|y\frac{dy}{dx}\right|=C$
$\Rightarrow B^2=AC$
$\Rightarrow$A, B, C are in GP