If $x=6t^2, y = \frac{6}{t^2}, $ then $\frac{d^2y}{dx^2}$ is equal to :

$-12\, t^4 $ $\frac{1}{3\, t^6}$ $\frac{1}{12\, t^5}$ $-\frac{1}{t^4}$

$\frac{1}{3\, t^6}$