(20 1 1? A model of Foshan) 1922 British physicist Aston won the nobel prize in chemistry for inventing mass spectrometer, isotope and mass spectrometry.

A, according to the deflection direction of charged particles in the magnetic field, the magnetic field direction is perpendicular to the paper surface, and according to the left-handed rule, the particles are positively charged. So, A is wrong.

B according to the left-handed rule, the Lorentz force on charged particles in the P 1P2 region is vertically upward, the electric field force direction is vertically downward, and the electric field intensity direction is vertically downward, so the P 1 polar plate of the speed selector is positively charged. Therefore, B is correct.

C, according to qvB=mv2r and r = mvqb, the larger r, the smaller the charge-mass ratio qm. So, C is wrong and D is right.

So choose BD.