A, according to the forward propagation of harmonic shear wave along the X axis, it is concluded that point P moves downward and point Q moves upward, and their periods are the same, so t = 2× (0.2s+1s) = 2.4s. Therefore, a is wrong.

B, λ =12cmv = λ t = 0.122.4m/s = 0.05m/s, so B is correct.

C. According to the analysis of the problem, the shortest time for the P particle to return to the equilibrium position is 0.2s, and the shortest time for the P particle to reach the trough from the equilibrium position is 0.6s. Therefore, when t=0.8s, the P point just reaches the trough, and the acceleration is maximum in the positive direction. So, C is correct.

D. The harmonic shear wave propagates along the X-axis, and the vibration direction of particle Q is perpendicular to the X-axis, and there is no displacement in the X-axis direction. So, d is wrong.

So choose BC