(20 14? As shown in the figure, the uniform magnetic field with the direction perpendicular to the paper is limited to the dashed area of an isosceles right triangle with the right angle L.

A, the magnetic field moves to the right at a constant speed, which is equivalent to moving to the left at a constant speed in framing; When the magnetic flux increases, according to the right-hand rule, the induced current is counterclockwise; When the magnetic flux decreases, the induced current is clockwise; Therefore, A is correct;

B when the cutting effective length is the longest, the induced electromotive force is the largest, which is: Em=BLv0, so the maximum induced current is:

Im=EmR=BLv？ 08Lr=Bv08r0, so B is correct;

C Assuming that the current is always equal to the maximum current, I=Bv08r0, the Joule heat generated in the metal frame when the magnetic field passes through the metal frame is:

Q=(I2R？ lv0)×2 = b2lv 20 16 r0； But the current is changing, so c is wrong;

D the maximum ampere force on the metal frame is FAm=BImL=B? Bv08r0？ L=B2Lv？ 08r0 Therefore, d is correct;

Therefore, ABD ..