A current I_1 carrying wire A B is placed near another long wire C...

Since the magnetic field, due to current through wire $C D$ at various locations on wire $A B$ is not uniform, therefore, the wire $A B$, carrying current $I_1$ is subjected to variable magnetic field due to which, neither the force nor the torque on the wire $A B$ will be zero. As a result of which the wire $A B$ will have both translational and rotational motions.