What would be the time period of oscillations?

Question

A vibrating system with mass 3 kg, stiffness 21 N-s/m and damper having damping coefficient 10 N-s/m. When an exciting force of magnitude 27 sin 2t is acting, what would be the time period of oscillations?                         

(a) 0.78 s                         (b) 1.56 s        

(c) 3.14 s                       (d) 3.89 s