B 1 ial1= b2ibl2 = f (1and 2 are subscripts, b1is the magnetic field generated by b and B2 is the magnetic field generated by a at b) ia & gtib =>b1<; B2，

Let the magnetic induction intensity of uniform magnetic field be B3.

Pairing a: b1ia l1= b3ia l1= f = > b1= B3 <; B2

Analysis of conductor B: The force it bears consists of two parts: the force A exerts on conductor B is F, and the force B exerted by an external uniform magnetic field is B3IBL2 (the direction of the external uniform magnetic field is the same as that of conductor A on conductor B).

Therefore, you should choose B.