Related Clues:

• Kind of copy
• U-235 or C-14, e.g.
• Element form
• C
• Heavy hydrogen, e.g.
• They share the same atomic number
• Type of copy
• Carbon-14, e.g.
• They may be radioactive
• It can help you get a date
• Carbon 13 or cobalt 60
• Chemist's study
• Chemical relative
• -- dioxide
• Elemental form