A, according to the topic, the metal bar moves in a straight line at a uniform speed, v=at. According to E=BLv=BLat, the induced electromotive force generated by metal bars increases uniformly. According to Faraday's law of electromagnetic induction, if the change rate of magnetic flux in the loop increases uniformly and the slope of φ-T image increases, φ-T image should be a curve, so A is wrong.

B, from the above analysis: E=BLat, and e = △φ△t, then: △φ△t = blat ∝ t, so B is correct.

C the average value of the induced electromotive force in the loop is: E = △φ△ t, and the induced current is:. I = ER+r，

The amount of charge passing through the metal bar q =. i△ t = △φ？ R+r=BLxR+r=BL？ 12at2R+r, which means q ∝ t2; Then the φ-T image should be a parabola, so C is wrong.

The potential difference between d, a and b is U=RR+rE=RR+rBLat∝t, so d is correct.

So: BD.