The angular speed of a fly wheel moving with uniform angular acceleration changes from $1200\text{rpm}$ to $3120\text{rpm}$ in $16\text{seconds}$. The angular acceleration in ${\text{rad/s}}^2$ is:

1. A
   $2\pi$
2. B
   $4\pi$
3. C
   $12\pi$
4. D
   $104\pi$