Mathematics is the most fascinating.

A, the actual motion can be divided into straight motion to the right and circular motion to the left;

For uniform linear motion, Lorentz force and gravity are balanced. According to the equilibrium conditions, there are: mg=qv0B,

Solution: v0=mgqB, so a is correct;

B, the period of uniform circular motion is: T=2πmqB, and after t= 12T=πmqB, the particles reach the lowest point, so b is correct;

C, D, uniform circular motion, Lorentz force provides centripetal force, according to Newton's second law, there are:

qv0B=mv02R

Solution: r = mv0qb

So the coordinate of the lowest point is: y=-2R=-2m2gq2B2, so c is correct;

Therefore: ABC ..