The angular displacement of a simple pendulum is given by $(t)={}_{m}cos(t+).$ If the pendulum is $45$ cm in length, and is given an angular speed $d\frac{}{d}t=3\mathrm{.}4ra\frac{d}{s}$ at time $t=0,$ when it is hanging vertically, what is ${}_m?$

A$)4\mathrm{.}6$ rad

B$)3\mathrm{.}4$ rad

C$)1\mathrm{.}4$ rad

D$)0\mathrm{.}73$ rad

E$)0\mathrm{.}45$ rad