Ans: A
By comparing the given figure to the graph of $y=\cos x$, we can found that the graph of $y=\cos x$ is enlarged along $y$-axis to $2$ times of its original and reflected about $x$-axis. Therefore, $a=-2$.
Consider the graph of $y=a\cos 2x^\circ$. The $2$ inside $\cos$ means contracting the graph of $y=\cos x$ along $x$-axis by $\dfrac{1}{2}$ times of its original. Therefore, $b=90$.
