Two blocks A and B of masses 2m and m, respectively are connected by a massless and inextensible string. 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 
 

• A

  $\text{ g, }\frac{\text{ g }}{\text{ 2 }}$

• B

  $\frac{\mathrm{g}}{2},\mathrm{g}$

• C

  g, g

• D

  $\frac{\mathrm{g}}{2},\frac{\mathrm{g}}{2}$