According to the meaning of the question, A is the superposition of valleys, B is the superposition of peaks and C is the superposition of peaks and valleys;

Point a and point a are the superposition of valleys and valleys, which are vibration strengthening points and always strengthen vibration. So, A is wrong.

Points B and B are the superposition of peaks, and they are vibration strengthening points, which always strengthen vibration. So, B is right and C is wrong.

Point D and point C are the superposition of peaks and valleys, which are vibration attenuation points, and the vibration is always weakened. So, d is wrong.

Therefore, choose: B.