At time $t=0,$ the current to a dc electric motor is reversed, resulting in an angular displacement of the motor shaft according to $=12t-24t^2,$ where $$ is in rad and $t$ is in $s.$ The angular speed of the motor shaft is zero at time $t=$

A$.0\mathrm{.}5s$

B$.0$ and $0\mathrm{.}5s$

C$.0\mathrm{.}75s$

D$.0$

E$.0\mathrm{.}25s$