If $x=5\, t^3,y =7\, t^4 ,$ then $\frac{d^2y}{dx^2}$ is equal to :

$\frac{28}{15}t$ $\frac{28}{15\, t^2}$ $\frac{14}{5\, t^2}$ $\frac{28}{225\, t^2 }$

$\frac{28}{225\, t^2 }$