Two collinear, short, metallic rods of the same diameter are in contact. The top of the upper rod is electrically heated, and the bottom of the lower rod is cooled with a fluid. The exposed surface of the rods are thermally insulated from the surroundings. Which rods have the smallest thermal contact resistance?
a.) Copper rods with smooth surfaces
b.) Copper rods with rough surfaces
c.) Carbon steel rods with smooth surface
d.) Carbon steel rods with rough surfaces