If $x=5t $ and $y=\frac{5}{t}$, then $\frac{d^2y}{dx^2}$ at $t=1$ is:

$\frac{2}{5}$ $-\frac{5}{2}$ $\frac{1}{5}$ $-\frac{2}{5}$

$\frac{2}{5}$