As shown in the figure, in the square area abcd, there is a uniform magnetic field with a direction perpendicular to the paper surface and a magnetic induction intensity of B, which is located in the square when t=0. Particles move in a uniform circle in a magnetic field, and the trajectory of particles whose initial velocity is parallel to the ad direction is as shown in the figure, and its center is O 1. Let the side length of a square be l, which is obtained from the geometric relationship:

sin∠OO 1k=L2L= 12…①

De: ∠OO 1k=π6

Then t = t0 = π 62 π t =112t ...

T=2πmqB…③

The solution is QM = π 6bt0; Therefore, B is correct;

From equation 2: T= 12t0, so a is wrong;

Because the initial velocity of particles is equal and the trajectory radius of all particles is equal, the particle with the longest trajectory has the largest central angle and the longest movement time in the magnetic field. If the initial velocity direction is opposite to the four vertices in the magnetic field, the arc length of the particles is not the longest, and the time is not the longest. So, C is right and D is wrong;

So: BC.