One of the causes of erosion is the third primary

the answer: Water, wind, gravity

Erosion is one of the most important processes occurring on the Earth's surface and results from a number of forces, including the third primary force. The third primary factor is a combination of water, wind, and gravity, which together can cause erosion by removing and transporting rocks and other materials from one place to another. Rainwater and melting snow can also penetrate the cracks and pores of rocks, causing them to break down and erode more easily. All of these forces combined can cause erosion, creating canyons, valleys and other geological features. Understanding the causes of corrosion helps us better protect our environment from the harmful effects of these forces.