A, the uniform motion of the coil indicates that Ampere force and gravity are in balance, so: mg = B2L2 VR.

Solution: v = mgrb2l2

When the cd edge just enters the magnetic field, it just moves in a straight line at a uniform speed. When the ef edge enters the magnetic field, the ampere force increases by four times, so the wire frame will slow down, and the speed at the final balance should be 14 of the initial speed. According to i=BLvR, the current should start with12;

So, A is wrong;

B. Voltage: UBA = IR4 = E4；;

Beginning: Uba = BLv4；;

When the ef edge enters the magnetic field, Uba=B? 2Lv4 = BLv2

The final equilibrium speed should be 14 of the initial speed, so: Uba=B? 2L？ 14v4=BLv8, so b is correct;

C, when the ef edge enters the magnetic field, the ampere force increases to 4 times, and the kinetic energy of the coil will decrease due to overcoming the resultant force to do work; The final equilibrium velocity should be 14 of the initial velocity, so the kinetic energy is 1 16, so c is correct;

D, due to overcoming the ampere force to do work, the mechanical energy is continuously reduced;

According to the functional relationship, the change of mechanical energy e with displacement x represents ampere force, that is, FA=|△E△x|, and the slope of the tangent line at this point on the E-x image should be constant (but not zero), then suddenly increase and then gradually decrease. So, d is wrong;

So: BC.