A $\mathrm{U}$-shaped conducting frame is fixed in space. A conducting rod $\mathrm{CD}$ lies at rest on the smooth frame as shown. The frame is in a uniform magnetic field $B_0,$ which is perpendicular to the plane of the frame. At the time $t=0$ , the magnitude of the magnetic field begins to change with time t as, $B=\frac{B_0}{1+kt},$ where k is a positive constant. For no current to be ever induced in the frame, the speed with which rod should be pulled starting from time $t=0$ is (the rod CD should be moved such that its velocity must lie in the plane of frame and perpendicular to rod $\mathrm{CD}$)