6. What is a force that opposes motion between two surfaces that are in contact? A. Friction B. Motion C. Velocity D. Acceleration 7. Which one of the following is a disadvantage of friction? A. It keeps a runner from slipping. B. It helps a person to write clearly. C. It wears away the surfaces in control. D. It stops a car from sliding off the road. 8. Snails produce mucus to help them move along the ground. How does the mucus help snails move A. Mucus is cold. B. Mucus leaves the trail. C. Mucus reduces friction. D. Mucus makes the snail lighter. 9. Which of the following shows the presence of friction? A. Cars continuously roll down the hills. B. Things fall off on smooth surface. C. People stay in place as they move. D. Cars stop when the driver steps on the brakes.​