A $0\mathrm{.}19-$m radius grinding wheel takes $2\mathrm{.}8$ s to speed up from $3\mathrm{.}0ra\frac{d}{s}$ to $12\mathrm{.}0ra\frac{d}{s}.$ What is the wheel's average angular acceleration?

$6\mathrm{.}4ra\frac{d}{s^2}$

$3\mathrm{.}2ra\frac{d}{s^2}$

$1\mathrm{.}1ra\frac{d}{s^2}$

$0\mathrm{.}6ra\frac{d}{s^2}$

$16\mathrm{.}9$ rada ${?}^2$