Two blocks of masses 3m and m m\,m are connected by a light, inextensible string that passes over a smooth, fixed pulley, as shown in the diagram.

The system is released from rest under gravity, ggg.
Assuming neither block reaches the pulley or the ground within the time interval ttt, which of the following expressions represents the magnitude of the total momentum of the two-block system at time ttt?
12mgt\frac{1}{2} mgt21mgt
mgtmgtmgt
2mgt2mgt2mgt
4mgt4mgt4mgt