# The motion of a particle is described by equation $x= a+bt^2$ where $a=15\;cm,b=3\;cm/s$ The instantaneous velocity at time 3 will be
$\begin{array}{1 1} (a) \;36\;cm/s \\ (b) \;18\;cm/s\\ (c) \;16\;cm/s\\ (d) \;32\;cm/s\end{array}$