As I Use Sandpaper on Some Rusty Metal,the Sandpaper Gets

Multiple Choice

As I use sandpaper on some rusty metal,the sandpaper gets hot because:

A) heat is flowing from the sandpaper into the metal.
B) heat is flowing from the metal into the sandpaper.
C) frictional processes increase the internal energy of the sandpaper.
D) heat is flowing from my hand into the sandpaper.