A murder victim is found slumped over his desk in his office. The bullet that killed him is found lodged in his chair. Investigators find footprints in the carpet and gather traces of silica used to make concrete from the footprints. Their prime suspect is a construction supervisor who recently had an argument with the victim. The suspect's construction firm regularly uses a brand of concrete with high concentrations of silica. On the victim's desk is a threatening letter from the suspect regarding a disagreement between them. The suspect's fingerprints are found on the letter. Which of these pieces of evidence is showing a class characteristic?
A. The fingerprints on the letter belonging to the suspect
B. The silica in the carpet being like the concrete that the suspect's company uses
C. The victim being killed with his own gun
D. The suspect being angry enough with the victim to threaten him