A massless and inextensible string connects two blocks A and B of masses 3m and m, respectively. The whole system is suspended by a massless spring, as shown in the figure. The magnitudes of acceleration of A and B immediately after the string is cut, are respectively:
      

1. $\frac{g}{3},g$

2. g, g

3. $\frac{g}{3},\frac{g}{3}$

4. $g,\frac{g}{3}$