If $x=4 t^2, y=8 t$, then $\frac{d^2 y}{d x^2}$ is:

$\frac{1}{t}$ $-\frac{1}{8 t}$ $-\frac{1}{8 t^3}$ $-\frac{1}{t^2}$

$-\frac{1}{8 t^3}$

The correct answer is Option (3) → $-\frac{1}{8 t^3}$