It is harder to walk on snow than on a road with sand because . . . . . . . . . .

• A. Ice is soft and spongy, whereas scone is hard.
• B. The force of friction between the foot and the ice is less than the force of friction between the ball and the foot.
• C. The force of friction on ice is less than that of a ball.
• D. Here the escape velocity of the atoms is less than their square mean root velocity.