Here's a problem to solve: Two pieces of the same metal are placed on a table. The pieces of metal have the same density, mass, and texture. Which characteristic must be the ONLY difference between the two samples?

Among the 4 following options, which is the correct answer?

- A: They have different dimensions.
 
- B: They have different boiling points.
 
- C: They are made up of a different type of matter.
 
- D: They contain a different amount of matter per unit volume.
 