The correct answer is: B. the friction between sand and feet is less than that between concrete and feet.
Sand is a loose material that is made up of tiny grains of rock. When you walk on sand, your feet sink into the sand and create a space between your foot and the ground. This space reduces the amount of friction between your foot and the ground, making it more difficult to walk.
Concrete is a hard material that is made up of cement, sand, and water. When you walk on concrete, your feet do not sink into the concrete and there is very little space between your foot and the ground. This means that there is more friction between your foot and the ground, making it easier to walk.
Option A is incorrect because sand is not necessarily softer than concrete. Sand can be soft or hard, depending on the type of sand and the amount of moisture it contains. Concrete is always hard.
Option C is incorrect because the friction between sand and feet is less than that between concrete and feet. This is why it is more difficult to walk on sand than on concrete.
Option D is incorrect because the sand is grainy but concrete is smooth. This does not affect the amount of friction between your foot and the ground.