When the metal bar is connected with I=kt from time t=0, it can be known from the left-hand rule that the direction of ampere force is perpendicular to the paper surface and makes it tightly press the guide rail, which will lead to the upward friction of the bar during the movement.

According to N=F=BIL, the pressure of MN rod on the guide rail is increasing, and the friction force f=μN is also increasing. According to Newton's second law, it is mg-μBIL=ma, I=kt, so it is a=g-μBLkmt, so the acceleration decreases. Since the speed and the acceleration direction are the same, an acceleration motion with reduced acceleration is performed.

When the sliding friction is equal to gravity, the acceleration is zero, the velocity reaches the maximum, and its kinetic energy is also the maximum. When the ampere force continues to increase, the acceleration direction will be vertical upward, and the acceleration is opposite to the speed direction, so the acceleration will slow down. According to the slope of the image of speed and time, it indicates the magnitude of acceleration, so B is correct.

So choose: B.