Iction between sliding surfaces is caused by

1. Adhesion force - Occurs when two surfaces have molecules that attracts each other hence creating bonds which require more energy to break.

2. Rough surfaces - This is also referred to as a sandpaper effect. It occurs when a rough surface prevents the other from moving smoothly by creating some barriers.

3. Deformations - One or both of the sliding materials may be smooth but have non-uniform edge which objects the other from moving smoothly.

substitution is the answer in this case