If $x=acos \theta , y = bsin \theta , $ then $\frac{d^2y}{dx^2}$ at $\theta =\frac{\pi }{2}$ is :

$-\frac{b}{a}$ $-\frac{b}{a^2}$ $\frac{a}{b}$ $\frac{a}{b^2}$

$-\frac{b}{a^2}$