Sudden increase in velocity between crust and mantle is evidence of what?

Correct answer: Change in chemical and physical properties

The direct answer is A: a sudden increase in seismic-wave velocity between the crust and mantle indicates a change in chemical and physical properties. Seismic waves travel differently through different materials. At a boundary, a sudden change in speed shows that the material below is not exactly the same as the material above. The crust–mantle boundary is called the Mohorovičić discontinuity, or Moho. Thus the evidence is a discontinuity and a change in composition, density and physical condition. Option A is correct because it explains why wave velocity changes. Option B concerns ocean-surface waves, not the interior Earth boundary. Option C concerns atmospheric temperature and cannot explain seismic velocity. Option D is a surface river feature and is unrelated to Earth’s internal layers. Exam cue: sudden seismic change means a boundary or discontinuity, not a weather or surface-landform event.