A

Step 1: Understand the role of friction in running. Frictional force is necessary to propel oneself forward while running. Without sufficient friction, the feet cannot push against the ground effectively.

Step 2: Consider the properties of sand. Sand is typically a loose granular material. When weight is applied, such as when a person runs, the sand shifts and creates less contact area between the foot and the ground than a solid surface, thereby reducing frictional force.

Step 3: Analyze how this affects movement. If there is inadequate friction, it is difficult for runners to maintain balance and effectively push off the ground. This requires more energy and can lead to fatigue more quickly compared to running on solid ground.

Conclusion: Thus, it is indeed challenging to run on sand due to insufficient frictional force.