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The unit of angular acceleration in the SI system is

(1) $N\text{\hspace{0.17em}}k{g}^{-1}$

(2) $m\text{\hspace{0.17em}}{s}^{-2}$

(3) $rad\text{\hspace{0.17em}}{s}^{-2}$

(4) $m\text{\hspace{0.17em}}k{g}^{-1}K$