The value of $b$ varies directly with the cube of $a$ and square of $c$. If $b$=125 when $a$=5 and $c$=25, what will be the value of $b$ if $a=2$ and $c=4$

Question

$\begin{array}{1 1} (a)\;16 \\ (b)\;8 \\ (c)\;2 \\(d)\;\text{none of these} \end{array} $