Concept:Humid regions have abundant rainfall, so running water is the main erosive force.
Explanation:Humid regions receive high rainfall throughout the year.
This rainfall forms streams and rivers that continuously erode the land.
Running water wears away soil and rock and transports the eroded material.
Fog has very little erosive power and is not a major agent.
Wind is more significant in arid areas, not humid regions.
Ice is more important in cold polar and mountainous regions.
Therefore, running water is the most important agent of erosion in humid regions.
Answer:D. Running water