A block of mass 2M is attached to a massless spring with spring-constant k. This block is connected to two other blocks of masses M and 2M using two massless pulleys and strings. The accelerations of the blocks are $$a_1, a_2$$ and $$a_3$$ as shown in the figure. The system is released from rest with the spring in its unstretched state. The maximum extension of the spring is $$x_0$$. Which of the following option(s) is/are correct?
[g is the acceleration due to gravity. Neglect friction]