B, as can be seen from the figure, the equilibrium positions of P and A are half a wavelength apart, and the vibration situation is always opposite, so when the particle P is at the lowest point, the particle A must reach the highest point. So, B is wrong.
According to mathematical knowledge, the distance between the equilibrium positions of A and B is equal to 1.3 λ. When the particle P is at the lowest point and A reaches the highest point, the particle B must be below the X axis, so D is correct.
So choose AD.