Two steel balls 2 kg and 4 kg mass, respectively, are pressed on the two ends of a spring, all pre-placed on a smooth surface. When released, the smaller ball moves with an acceleration of 2 m/s2. The simultaneous acceleration of the bigger ball will be
(a) 0.5 m/s2 (b) 1 m/s2
(c) 2 m/s2 (d) 4 m/s2