Concept:Coastal erosion is caused mainly by the continual force of moving seawater, and waves are the most powerful and constant agent.
Explanation:Sea waves strike the coast with high energy, breaking and wearing away rocks through processes like hydraulic action, corrasion, and attrition.
Sea currents and tides mainly transport and redistribute sediment, so their erosive effect is weaker and less direct than that of waves.
Underground water acts below the surface and is not a major agent of coastal erosion.
Therefore, the continuous pounding of sea waves makes them the most effective agent.
Answer:B. sea waves