A, the final electron is in equilibrium under the action of electric field force and Lorentz force. Let the length, width and height of Hall element be A, B and C respectively, where qUc=qvB, and the microscopic expression of current is I=nqvS=nqvbc, so the larger U = BinqB. B, the greater the potential difference between the upper and lower surfaces. Therefore, a.

The greater the values of b and k, the greater the variation of b with z according to the magnetic induction intensity B=B0+kz, and the greater the variation of u with z according to U = BINQB, that is, the higher the sensitivity of the sensor (△U△z), so b is correct.

C. free electrons move in the hall element. According to the left-handed rule, electrons are biased towards the lower surface, so the upper surface potential is high. So, C is wrong.

So choose ab.